RICERCA ITALIANA     

Polyesters functional properties optimization for packaging applications by morphology control, nanofillers and nanoreinforced coatings

Università degli Studi di Palermo

Abstract

The improvement of transport properties of polymers used in food packaging applications based on polyethylene terephthalate (PET) will be undertaken following different strategies such as:
· copolymerization with a rigid-chain polyester,
· blending with an Organoclay type commercial nanofiller, or
· blending with a new generation nanofiller like POSS (Polyhedral Oligomeric Silsesquioxane), with promising development perspective due to their plausible low production cost or finally
· with coatings with polyurethane adhesives in their turn reinforced by nanofillers.
These objectives will be carried out through the control of the primary structure in the copolymers and by the compatibilization of the nanoreinforcements using both 2D types (organoclays) and 3D types (POSS). New melt blending methods that will allow to incorporate greater quantitites of nanofillers will be investigated. The interactions with the rheological characterizations, used as an instrument for analyzing the structure formed, will permit to optimize the operating conditions. A complete macro- and microscopic characterization of the structure will allow to analyze the best processing conditions to be used for obtaining a satisfactory exploitation of the nanofillers.
The use of a solidification technique simulating the conditions encountered in polymer processing will allow to highlight the interactions between the morphology developed and the cooling conditions as well as its dependence on the nature of the polymer used, on its primary structure and the on the additives incorporated.
The possibility to obtain substantially homogeneous samples in an interval of cooling rates till above 1000°C/s permits to apply the macroscopic characterization techniques and therefore to render plausible the correlation between the transport properties and the morphology developed during solidification.
It is thus possible to analyze also the eventual sinergy deriving from the onset of the morphologies stabilized by the presence of nanofillers such as for instance in the case of the formation of metastable phases whose interaction with material transport is still under debate.
The molecular simulation will be used to provide indications on the synthesis of compatibilizers and to predict the transport properties. The comparison between these predictions and the experimental results will hence allow to shade light on the fundamental role of morpholgy in determining the transport properties.
The different techniques utilized for improving the transport properties will permit to draw up a classification based on the typical prerequisites necessary in materials destined for food packaging applications <<<

Principal Investigator

Stefano PICCAROLO Università degli Studi di PALERMO

Research Objectives

The main goal of the project is to improve the barrier properties of materials used in food packaging applications. This project is centred on polyethylene terephthalate, a polymer that is widely used and well established in the sector which also enjoys well developed processing technologies.
The same sectors that already use PET will benefit from the strategies followed in this project. They will be able to broaden the horizons of the utilization of this polymer by a better control of the primary structure considering the possibility of using nanofillers and analyzing the optimum conditions of the transformation process.
An important parameter, especially for films used in food packaging, is low permeability to oxygen since this is the condition that determines the capacity of the film to preserve the aliment unaltered although in recent years, the demand also includes heat resistant packaging materials that can be directly used in ovens or boiled in water.
The reduction of the permeability to gases and vapour implies the possibility of using lower material thickness, guaranteeing a sufficient level of barrier or the possibility of using the same thickness however considerably increasing the shelf-life of the packed food product. Besides, if one considers the fact that the use of nanofillers does not alter the transparency of the polymeric matrix and also the real possibility that it offers in improving the mechanical properties, this means having a material that is competitive even with those obtained by the most sophisticated technologies used in the sector. In fact, the approaches often proposed to increase the barrier properties include the use of highly expensive plants for the production of the monomers or polymers that meet the requirements to be used in direct contact with aliments. Actually, use is made of multilayer processing technologies in which high barrier material is included between layers of PET thus obtaining structures having optimum permeability and mechanical properties.
This fundamental objective will be pursued using a multiplicity of strategies aimed at quantifying and comparing the variations in material transport properties resulting from:
a) copolymerization with a rigid-chain polyester
b) blending with an organoclay type commercial nanofiller
c) blending with a new generation nanofiller like POSS (Polyhedral Oligomeric Silsesquioxane), which shows good development perspective due to the plausible contained cost of production
d) coating with polyurethanes adhesives in their turn reinforced with nanofillers

These strategies will conduct to the identification of different quality grades as a function of the specific characteristics in the food packaging application. It will therefore be possible to:
· identify the conditions that will optimize the required properties
· establish a scale between the investigated materials accounting for the summa of the examined prerequisites.
· highlight the conditions that unite a good balance of the examined properties to a reasonable cost for the applications of interest, characterized in general by a moderate added value.
· point out the formulations that, even if scientifically plausible, lead viceversa to a set of characteristics of poor interest

These objectives will be pursued with intermediate results of both scientific and practical interest, in particolar:
· techniques will be realized for synthesizing and identifying compatibilizers that will be used for improving the structure of the PET/PEN copolymers in the solid state
· the interactions with the results obtained from molecular simulation will allow to identify the optimum blending conditions and the most appropriate formulation conditions that will enhance the formation of blocks rather than the tendency to randomization.
· Likewise, other compatibilizers that will improve the adhesion between POSS and PET will be identified
· Similarly compatibilizers between polyurethane coatings and POSS will be identified
· The molecular simulation will assume a fundamental role in the identification of the most appropriate composites in the polymer-compatibilizer-nanofiller ternary blends
· The simulation will furnish useful indications on the transport properties and will also permit the interpretation of the results obtained from the characterizations
· The tendency deriving from intensive blending to which blends with nanofillers are generally subjected to can be recognized by extending these approaches to the situations in which the system is exposed to a force field.
· These techniques will be further accompanied by campaigns of characterizations of the rheological properties of the scrutinized systems with the aim of investigating the most appropriate blending conditions for an efficient exploitation of the nanofiller.
· Sample-wise characterizations by transmission electronic microscopy will allow to analyze the morphology resulting from the blending operation
· Characterization of the mechanical properties will thus permit to evaluate the obtained blend with respect to the prerequisites posed for packaging materials.
· The relationship between the morphology and the operating conditions used in industrial productions will be examined by a fast cooling technique similar to the continuous ones used in metallurgy (CCT)
· Homogeneous films that simulate the solidification conditions in polymer processing will be obtained
· The peculiar characteristic, i.e., the structural homogeneity, will make possible mass characterization with the assumption that these are representative of the solidification conditions as well as the grade of the material examined .
· It will be possible for the first time to evaluate the transport properties at a controlled morphology both for the PET/PEN copolymers and for the nanocomposites.
· The characterizations of the structure formed using WAXD, densimetry and microhardness will hence be correlated not only generically to the material grade used but also to the solidification conditions.
· This situation will allow to put in evidence the peculiarities connected with the growth of the morpholgies induced not so much and not only by the material grade but also and above all by the solidification conditions.
· the interactions between the liquid-crystalline character of the polyester with semirigid chain with the morphology developed will be highlighted
· The role of the geometry (shape) of the nanoreiforcements on the formations and stabilities of the matastable phases that form in drastic solidification conditions will be highlighted
· It will be possible to make one more step in comprehending the relationship between the transport properties and the morphologies
· The characterization of the latter ones will provide indications for the optimization of the primary structure of PET/PEN based copolymers by blending, and the incorporation of nanofillers and for the best compatibilization methods.
· It will be possible to detect the interactions at the interphases through nanoindentation measurements at increasing loads
· The realization of this technique by itself will be an additional method for the characterization of local mechanical properties at a nanometric scale.
· Multiple possibilities result from the use of coatings or adhesives that can be applied as thin layers on the already formed materials.
· The interaction between molecular simulation, compatibilizer design and the transport property will be extended to polyurethane, a typical polymer used in these circumstances. <<<

First Results

In particular, making reference to the scheme in Fig. 1 and to the synoptic and temporary picture of the undertaken tasks (Gantt's diagram) in Fig. 2, the actions (with the list of expected results following each task) will be:

A.a PET+ PEN copolymers
This activity will focus both on the synthesis of innovative PET-PEN copolymers and on the preparation of systems of PET-POSS, PEN-POSS and on the functionalizing of POSS using polyols. The synthesis of PET/PEN copolymers, in particular, includes block as well as random copolymers. As regards the second case, techniques of polycondensation will be used modifying both the composition and the degree of randomization.
The block copolymers instead will be synthesized starting from commercial polymers. The synthesis will be carried out in two successive stages. In the first stage the functionalization of the base polymers with reactive end groups will be accomplished and in the second stage the functionalized polymers will be made to react in order to form the block copolymers. Different compositions, block lengths and end groups will be obtained.
· Synthesis of random PET-PEN copolymers with various percentage compositions and different degrees of randomness .
· Synthesis of block PET-PEN copolymers with various percentage compositions, block length and end groups.

A.b Melt Compounding: PET+ PEN/ compatibil PET +PEN
This activity includes the production of extruded films from the melt blended PET/PEN as well as from others prepared by blending these with those synthesized under the activity A.a. The starting materials will be properly dried and blended in dual screw extruders in a controlled environment from which will be produced films available for following characterizations
· Preparation of films from the PET/PEN blends having their structure determined by the conditions of blending and the formulations produced in A.a.

A.c Melt Compounding: PET+ commercial nanofiller (Organoclay)
Extruded films will be produced following the melt blending of PET with an appropriately compatibilzed commercial nanofiller (Organoclay). The extruded film will be obtained by melt blending of the oligomeric PET with organoclay in a bi-screw extruder in a controlled environment followed by increasing the molecular weight through the various procedures such as solid state polymerization, chain extension, blending with a higher molecular weight polymer.
· Production of blends of the oliogomeric PETand Organoclay for rheological characterization by using biscrew extruder and successivly upgraded either by post copolymerization or by using chain extenders or even by blending with a higher Mn PET,
· Preparation of PET /Organoclay films for the characterization of the structure formed, the mechanical and transport properties.

B.a Molecular Simulation for copolymers
Through a study of molecular dynamics and Monte Carlo simulation on the system PET-PEN (rigid chain) copolymer, it will be possible to determine the behaviour of the system phases, the solubility and the diffusion of gases in the matrix. Whereas regarding the block copolymers, it will also be possible to carry out morphological studies of mesoscale??? in order to determine surfaces of iso-density and the phase behaviour at a mesoscopic level.
· Report on the modelling procedure for the copolymer systems and on the results obtained for the polymers of interest
· Report on the modeling methods and procedures at a mesoscale level for the copolymers and on the results obtained in the mesoscale simulation.

B.b Molecular simulation for the ternary polymer-compatibilizer-nanofiller system
From a study of molecular dynamics on the ternary system, it is possible to determine the energy in play between the molecules of the polymer, the compatibilizer and the solid interface of the nanofiller and therefore to make a comparison of the compatibilization capacity of the different chemical substances. Besides, again through the use of NPT molecular dynamics simulation, it will be possible to determine the position of the platelet of the commercial nanofiller as a function of the compatibilizer employed. In the caseof using a commercial nanofiller, which is already comatibilized, it will remain possible to determine the dimension of the spacing between the agglomerates of the nanofiller in the polymer matrix.
· Report on the calculation procedures of the bond energies for the polymer - compatibilizer - nanofiller systems at atomistic level.
· Report on the calculation procedures of the mesoscale parameters starting from the atomistic simulation for all of the involved components.
· Study of the bond energy for the polymer - commercial nanofiller system.

C.a Structure of the PET/PEN copolymers
The NMR technique will serve to determine the structure of the copolymers and the end groups present on the block copolymers. The GPC technique will be employed in the determination of the molecular weights of the obtained copolymers wheras their thermal properties will be analyzed by using DSC. Evaluation of the thermal stability properties will be conducted on some of the copolymers by using DSC and TGA.
· Report on the structural charcteristics and the basic properties of the produced copolymers: copolymer composition , degree of randomness (for the random copoymers), block length (for the block copoymers), average molecular weight and the molecular weight disribution curve, the glass transition as well as the fusion temperatures.
· Report on the thermal stabilitycharcteristcis .

C.c PET+clay Rheology
One of the critical points in the modification of the compounding for films is connected to the variation of the rheological properties, their stability and repeatability. Dynamic mechanical tests made in the linear viscoelastic region will give information on the structure of the nanocomposites (percolating structures, degree of exfoliation/intercalation/dispersion of nanofiller).
· Flow curves for the formulations of A.b at differnt temperatures.
· Dynamic mechanical tests for the determination of the dependence of the modulus and the dynamic viscosity on the frequency and temperature.

C.e Solidification of PET+PEN
The material will be accurately dried; the melt film will then be maintained in an inert environment generally for a short time due to its low heat capacity. After the solidification, the film will be stored at low temperature before being subjected to successive characterizations [37]. The charcterizations will be carried out with wide angle x-ray diffractometry (WAXD) and density measurements both methods representing the crystallinity in the stable phases developed. Examination with scanning calorimetry will furnish indications on the amount of the amorphous rigid phase present[48].
Samples with homogeneous morphology in a range of cooling rates ranging from 0.1 to above 1000°C/s for transport property tests (pt C.o)
Samples with homogeneous morphology in a range of cooling rates ranging from 0.1 to above 1000°C/s for macroscopic (pt C.h) and microscopic (pt C.i) mechanical tests.
Dependence of the structure formed (from WAXD and densimetry) on the cooling rate as a function of the primary structureof the copolymer as detrmined by the activities A.a and A.b.
Dependence of the rigid amorphous phase by scanning calorimetry.

C.f Solidification of PET+nanofiller
In this case, characterization of the morphology, can be indirectly obtained from microhardness measurements and its mapping, whereas densimetry and WAXD can only give partial information. In the experimented drastic solidification conditions, it is possible to observe the onset of metastable phases due to the topological constraint induced by the presence of exfoliated nanofillers that superimpose on the complex topology already present in the molten polymers. In spite of this situation, that could lead to the formation of nanocrystals with limited stability due to their contained size[49], it is however known that exfoliated nanofillers will bring to a crystallization confined with a limited order to small dimensions. This result, about which there are already hints in the literature [50], is relevant from the point of view of transformation processes. Although it may provoke a reduction of crystallinity in stable phases, it should equally lead to the increase of metastable ones thus improving the long term overall stability by reducing the variation of crystallinity in the finished product due to thermal gradients.
The other products of this activity are similar to those described in the previous point apart from the following :
Estimation of the dependence of the crystallinity in the metastable phases on the cooling rate
Comparison of the structural stability of the nanoreinforced and the standard PET at a temperature greater than the Tg.

C.h Macroscopic mechanical properties
The measurement of the macroscopic mechanical properties makes part of the characterization to be conducted on the polyester/nanofiller blends that are more interesting from the application point of view. So, in the final phase of the project, tensile tests will be conducted on a limited number of samples. It will be important, in particular, to evaluate not only the expected increase in modulus and resistance but it is also more important to verify that the deformation at rupture is not excessively reduced. In fact, the brittleness of the nanocomposite with respect to the sheer matrix is not easily acceptable in view of its destination for flexible packaging applications.
· Mechanical properties of the films with different compositions produced at pts. A.b, A.c, and comparison with the material without nanofiller giving particular reference to the mechanical properties at high deformation.

C.i Morphology AFM PET+PEN/ + nanofiller
Finally, Atomic Force Microscopy (AFM) will be utilized as a nanoindentation technique for the evaluation of the distribution of the properties associated with the hardness and the elastic modulus at different scales according to the load and the sensor used [51] and eventually uniting this with morphological information [52]. By modifying the interaction between the sensor and the substrate, it will be possible to analyze the degree of dispersion of the nanoreinforcement not only from topographical information but also from the localized mechanical response [51].
· Realization of a new technique for the measurement of the mechanical properties at a nanometric level
· Comparison of the nanometric mechanical properties with the mass properties obtained from C.h
· The dimensional scale in which the material can be considered as a continuum
· Dishomogeneity in the mechanical behaviour at the interface with the filler in dependence on its nature and that of the compatibilizer used

C.n Morphology TEM
TEM analysis will be utilized as a confirmation of the data obtained by WAXD and eventually to evaluate whether or not there is periodicity between the lamellae of the nanoclay not detectable by at angles lower than 1.5° with the WAXD technique. Moreover, the anlysis by TEM will provide information only on a limited portion of the sample and therefore TEM alone cannot give sufficient information. In view of the necessary scale-up of the blending processes, it is always advisable to consider as techniques of preference those that have the capacity to provide the average properties of the nanocomposite.
· Sampling on the materials produced at pts A.b, A.c for the determination of the degree of exfoliation or dispersion of the nanofiller.

C.o Transport properties of PET + PEN +nanofiller
The gas and vapour transport properties in the nanoreinforced PET (PET/clay) and in PET/PEN copolymers will be determined and hence correlated with the structural and morphological information available on the samples. Permeability tests to gases as well as gas and water vapour absorption measurements will be conducted which will then be analyzed using available models for the interpretation of mass transport in nanocomposites also analyzing the aspects related to the thermodynamics of gas absorption..
· The gas and vapour transport properties of the copolymers and their formulations produced in A.b.
· The gas and vapour transport properties of the nanocompositesin PET with commercial Organoclay produced in A.c
· Correlation of the molecular predictions obtained in B.a
· Interpretation of the results based on the available models in literature
· Conparison between the results obtained from the morphological charcterization and those inferred from the models.This part of the program is articulated in the following activities as reassumed in the block diagram of Fig.1 and the Gantt diagram of Fig.2.

A.d Melt Compounding: Coatings and adhesives PU+clay
The program intends a preliminary selection of a commercial formulation of an adhesive and coating to be used in food packaging. One of the two components to be used in formulation will be modified by the addition of nanostructured cationic clay, in their turn modified with different aluminium salts having chains ranging between C12 and C18. The blending will be carried out at elevated shear stress on the oligomer polyol by using a mixer at a controlled temperature and rotation speed.
· at least one formulation with nanodispersed clay will be available to be subjected to further characterizations

A.e Compatibilization of POSS for PET
Appropriately functionalized POSS, as in those characterized by the presence of pendant chains with ester group, will be used for the preparation of PET-POSS copolymers. The effect of organic substituents (present on the surface of POSS) on the solubility and compatibility characteristics with PET will also be evaluated in this phase. Selected POSS will be therefore used both for polymerization of PET-POSS copolymers starting from the basic monomers as well as for the reactive blending of functionalized POSS with the PET copolymers.
· Synthesis of functionalized POSS that is compatible with PET
· Report on the characteristics of the miscibility and compatibility of the different POSS with respect to PET
· Synthesis of PET-POSS copolymers

A.f Melt Compounding of PET and compatibilized POSS
This activity comprises of melt compounding of PET with compatibilized POSS followed by production of films by extrusion. The extruded film will be prepared by direct melt blending in a dualscrew extruder under a controlled environment. A series of preliminary tests, which probabily require a three months tme, will be carried out giving due space to the interactions with the rheological charcterizations of pt. C.d.
· PET /POSS films for the characterizations of the structure formed, the mechanical and the
transport prorperties.

A.g Compatibilization of POSS for PU
Functionalized POSS that are compatible with the selected polyol systems will be prepared. In particular the synthesis of POSS carrying such groups as isocyanates by polymerization and/or grafting with the polyol will be realized. The effect of the organic substituents on the miscibility property of POSS with respect to the prechosen polyol will also be studied.
· Synthesis of functionalized POSS that are compatible with the polyol systems utalizable in the preparation of adhesives and/or polyurethane based coatings.
· Report on the miscibility of POSS with respect to the polyole systems as a funcion of the variation of the organic substituents of POSS.

A.h Melt Compounding : coatings and PU+compatibilzed POSS adhesives
Blending techniques such as mechanical and ultrasonic mixers will be used. aranno The control of the blending condition will be obtained mainly by varying the time and the temperature.
· at least one formulation with POSS will be available destined for further characterizations

B.c: Molecular simulation of the the ternary polymer - compatibilizer - POSS system
The molecular dynamics studies already illustrated in Phase 1 will be here extended to the case of a 3D filler and POSS. They will give indications for the choice of the compatibilizer

B.d Molecular simulation for the polymeric systems under applied stress
Non Equilibrium Molecular Dynamics (NEMD) studies will permit to charcterize the material also when it is under an applied stress and in thermal gradients.
Besides to the results already put in evidence, there will be produced a:
· Report on the procedure of modelling in non equilibrium conditions and on the results obtained from the simulation of the systems confined between two walls

C.b Structure of compatibilized + PET/PEN/PU
The structure of of the POSS synthesized in phase A.g will be charcterized by means of the NMR technique. The verification refers particularly to both the insertion of the pendent groups necessary for the copolymerization with the polyols as well as the presence of organic substituents that control the miscibility of of the POSS.
· Report on the analysis of the synthesized POSS.

C.d Rheology of PET+POSS
Also in this case, rheologic tests in shear under steady state and oscillatory conditions will be carried out on the material in the melt state after the blending stage in the extrusor with the aim of htghlighting the the processing conditions and the compatibilizers that lead to a significant increase in the rheologic properties. The differences in behaviour between the compatibilized 3D nanofiller with respect to the organoclay will also be evidenced.
· Flow curves of the formulations of pt A.e at different temperatures
· Stability and evolution of the rheologic properties as a function of the time in which the material gets stressed during the course of the measuremnet.

C.g Solidification of PET+compatibilized POSS
Studies on the solidifiaction of polymers reinforced with POSS are not known so far. The same is true about the influence of POSS on solidification in processing conditions. The peculiarities of these nanofillers with respect to the organoclays, about which is discussed already, however provides indications on the expected prospectives. In this regard, the elevated connectivity estabilished by compatibilization of the nanofiller with the polymer brings, as already cited, a further limitation to the mobility apparently creating a real and proper network.
The products of this activity are similar to those decribed at pts C.e-C.f
Comparison of the structural stability of PET nanoreinforced with 2D with respect to that reinforced with the 3D as a function of the cooling rate

C.l PET + POSS Morphology by AFM
The nanoindentation technique already described at pt. C.i and applied on the reinforced PET (with commercial nanofiller), will also be used in this very similar situation which however is charcterized by peculiarities that promise some interesting deductions about the different connectivity that takes place in the present case.
· Comparison of the nanometric and the mass mechanical properties
· The dimensional scale in which the material can be considered as a continuum
· Dishomogeneity in the mechanical behaviour at the interface with the filler in dependence on its nature and that of the compatibilizer used

C.m Morfologia AFM rivestimenti con nanofiller
In the case of coatings and very thin films, the unique way for the determination of the mechanical properties is certainly the nanoindentation technique. AFM also in this case gives the possibility to to apply stresses independent of the substrate. Thus modifying the interaction between the tip and the material, also provides indications about the influence of the substrate on the mechanical properties of thin films [53] and therefore on the interactions and the adhesion between the substrate and the coating.
· Distribution of the mechanical properties of of the coatings in relation with the compatibilizer employed and as a function of the modality of between the matrix and the nanofiller.
· Interacions between the coating and the substrate as a fuction of the applied load.

C.p Transport Properties of PET + POSS
C.q Transport Properties of PET + coating + nanofiller
The gas and vapour transport properties in nanoreinforced PET (PET/POSS) as well as those of the adhesives and coatings nanoreinforced with organoclay and POSS will be determined and correlated with the structural and morphological information available on the samples. Measurements of permeability to gases as well as measurements of gas and water vapour absorption will be done which will then be analyzed by using the available models for the interpretation of mass transport in nanocompsites. The results will be compared with the predicions of the molecular simulations of pts. B.a e B.b
· Gas and vapour transport properties of the nanocomposites in PET compatibilized with POSS produced at pt. A.e
· Gas and vapour transport properties of the nanocomposites in PU and compatibilized with POSS produced at pt. A.g
· Correlation with the molecular prediction obtained at pts. B.c and B.d
· Interpretation of the results on the basis of available models in literature
· Comparison between the results obtained from the morpholgical characterizations and those inferred from the models <<<

Timescale

24 months

National and international background

Polyethylenterephthalate (PET) is the material of choice to produce containers for mineral water and soft drinks. Recently it is being introduced for packaging of more sensitive products, e.g. fruit juices, beer , wine, dry food, sauce, milk. The most important factors affecting the shelf life of these products are the microbiological sensitivity, the action of light and the permeation of gases and aromas through the package layer. Another important aspect is the possibility for the material to withstand thermal and sterilizing treatments necessary to guarantee the requirements of cleanness and those imposed by container filling under aseptic conditions.
In fact the need for higher barrier in PET has been recognized for many years and numerous PET backbone modifications have been accomplished to enhance barrier characterized by high cost and investment required to built new monomer and polymer plants and to obtain food contact regulatory clearances for new polymers. Technological solutions with active and passive barrier are known through multilayer processing techniques, where high-barrier polymers are sandwiched between PET layers [1].
PET based nanocomposites able of displaying barrier improvement from dispersion of low to modest levels (generally <6-8%) of relatively inexpensive clay materials and of being processable with conventional stretch blow molding processing equipment. Another alternative is to develop nanocomposite coatings and adhesives which can be respectively deposited on the surface of finished containers or films or used to laminate PET films.
The high aspect ratio of polymer-layered silicate nanocomposites (PLSN) (50-1000) and their high surface area (around 750 m2/g) promotes, in the cases of a good dispersion and exfoliation of the layered silicates, a significant improvement of mechanical properties (Young's modulus, toughness and strength) [2,3]. Due to the very high number of individual nanolayers of phyllosilicates particles, a small percentage of nanofiller (2-6% by weight) allows a very high interface area between polymer and silicates [4].
Smectite group of clay minerals (e.g. montmorillonite, saponite and hectorite) have been used to prepare nanocomposites due to their excellent intercalation capabilities. Organized in a sequence of nanolayers with lateral dimension of 200-2000 nm and a thickness of about 1 nm [5] exchange of the cations in the gallery space may be used for nanocomposites formation [6].
While in conventional microcomposites the clay layers adopt an aggregated morphology, in intercalated nanocomposites one or few molecular layers of polymer are inserted into the clay galleries. Exfoliated nanocomposites, formed when the silicate nanolayers are individually dispersed in the polymer matrix, show greater phase homogeneity than intercalated nanocomposites and each nanolayer contributes fully to interfacial interactions with the matrix supplying and effective improvement of the reinforcement and of other performance properties. Clays are treated to facilitate subsequent exfoliation and are available in the form of organoclays [7-10].
Actually, a PLSN will be partially intercalated and partially exfoliated. In general nanocomposites morphology is characterized by the level of ‘swelling' of the interlayer distance, the orientation of nanolayers and their level of dispersion.
Characterization of nanocomposites morphology is performed mainly by X-ray diffraction and Transmission Electron Microscopy. From the first technique it is possible to gather information on the interlayer spacing, on the relative orientation of the nanolayers while the second technique supplies information on the local structure of the sample and on its local interlayer spacing.
Very small amounts of exfoliated clay in PET, typically < 5wt% ash, give significant barrier improvement to gases (oxygen, carbon dioxide) and water vapor [11]. Achieving the degree of exfoliation desired requires the selection and optimization of many variables including choice of the matrix, process of incorporating clays, choice of clay, clay treatment, optional use of dispersing aids. Melt compounding is a desirable method of incorporating clays into polymers since it allows to use the PET as it is currently produced, it is a convenient and flexible process capable of producing a variety of formulations on a variety of product volume scales and the high shear environment of the melt extruder may permit the incorporation of significantly higher concentrations of clay compared with the concentrations that can be achieved by an in situ polymerization process. There are very few examples in the literature of preparation of PET nanocomposites by melt compounding [12,13] and it is worth noting that degradation upon melt compounding of PET is severe. A significant improvement can be obtained by adding oligomeric and polymeric materials to organoclays to further expand the basal spacing between the clay layers and further improve exfoliation during melt compounding with polymers [14]. Addition of expanding agents can be performed during melt compounding
Nanocomposites coatings could be used in a multilayer approach to improve the gas barrier properties of PET. A very thin layer of a nanocomposites coating can be applied after forming the article or continuosly deposited on thin PET films. In the literature several examples have been reported related to the use of nanoreinforced thermosets to improve barrier properties of PET films and containers [15,16]. It is reported that these highly loaded nanocomposites coatings have very high gas barrier, and thus it is understandable that a considerable amount of research is being devoted to these nanocomposites coatings. Similar approaches can be adopted in the development of nanocomposites adhesives. A class of polymers to be considered for such applications is that of polyurethanes produced by reaction of polyols and diisocyanates. A proper tailoring of the molecular structure of the two components should allow a good exfoliation of layered clays prior to crosslinking reaction (in fact exfoliation is likely to occur in the polyol phase before mixing it with the diiscyanate fraction).
A new trend in polymeric materials consists in combining inorganic and organic polymers structures in order to obtain "ceramers" with the typical properties of ceramics (high temperature resistance, stiffness etc.) and plastics (easy processability, plasticity, low density etc.). Polyhedral oligomeric silsesquioxanes (POSS), three dimensional compounds with general formula (RSiO1,5)n where n is an even number and R is an organic group, are the smallest silica particles available and can be considered as a nanometric distribution of inorganic polymer inside the organic matrix. In comparison with silica and clays, they can be easily functionalised with covalently bonded reactive groups which can be copolymerised or inserted in the macromolecule by grafting, leading to chemically bonded hybrid materials ("macromers").
The enhancement of physical properties of polymer matrices as consequence of POSS incorporation is directly related to the POSS's ability to control the motions of polymer chains without loosing the processability and the mechanical properties. This is due to the nanometric size of the POSS molecules which are nearly equivalent in size to most polymer segments and coils. The development of efficient synthetic protocols for preparation of POSS molecules, paved the way to the commercialization of such reagents which can now be used effectively to improve physical and technological properties of polymers. In connection to packaging applications POSS give rise to gas permoselectivity where they can be used as modulators of barrier properties with particular reference to selectivity enhancement, enhancement of oxidation resistance, the reduction of viscosity and improvement of mechanical properties [17]. Synthesis, use and processing of POSS based nanocomposites is well documented [18-27].
In the POSS hybrids the inorganic-organic interface contact is not very large, but the most important feature is the possibility of tuning nature and strength of the interaction of the inorganic molecules with the polymeric matrix, ranging from the chemical affinity (weak interaction) to the covalent bonding of the organic chains to the inorganic cage. The study of the surface properties of these siliceous materials is fundamental for understanding the nature of the interactions at the organic/inorganic interface, probably deeply involved in the formation of inorganic-organic hybrids. The surface is, in fact, the interface of the solid with the external environment and reflects only partially the features of the bulk. Thus the surface of the material can be strategic in determining the viscoelastic behaviour of the material, the deformation mechanisms responsible of the material behaviour at high strain levels (yielding and post-yielding) and the material fracture toughness.
Another important aspect that has not yet been studied is the potential environmental impact of these new materials which is, however, essential in order to evaluate their applications. In particular, at the end of life thermoplastic ceramers could be recycled in a "close-loop" to reproduce the same element which is generally impossible with traditional composites.
Further development of this promising technology requires a deeper understanding of the problems related to the use of different polymer matrices of technological interest, with the aim at optimising both the processing of nanocomposite or hybrid materials and their final properties. In particular the class of POSS-containing polyesters has not received yet almost any attention, although it deserves a detailed and accurate investigation for the many potential benefits linked to their field of utilization.

One of the most important functional property of materials for packaging and of nanocomposites in particular is the barrier to mass transport of gases and vapors. Several models are available for interpretation of mass transport properties of nanocomposites. One of the first models is that proposed by Nielsen [28] which was originally proposed for microcomposites but has been successfully applied also in the case of various nanocomposites systems [29,30]. This model assume the nanofiller particle as being small layers of cylindrical or squared shape and totally impermeable to gas and vapor molecules. Moreover these layers are assumed to be all oriented in the same direction (parallel to the surface of the polymer sheet) and uniformly dispersed in the matrix. Tortuosity of diffusive path and reduction of transverse area exposed to the mass flux are the main factors responsible for reduction of effective permeability of the nanocomposites as compared to the neat polymer.
More recently Bharadwaj introduced several modification of the above discussed model [31] to take into proper account two important factors: the relative orientation of nanolayers and their level of aggregation. G.W. Beall [32] has proposed further modifications to Nielsen model taking into account the presence of different zones in the polymer characterized by different mobilities due to the constraints imposed by the presence of nanolayer. These models refer to steady state permeation processes and cannot be applied when dealing with transient phenomena. Other models, which cannot be classified as modifications of Nielsen models have also been proposed [33,34].

Another latitude for control of morphology and interface is offered by the peculiar crystallization behavior of PET, a slow crystallizing polymer which even in when apparently amorphous is tough. This behaviour is due to the micro or latent crystallinity induced by chain rigidity [35]. Not all amorphous PET is amorphous, PET crystallization can give rise to different levels of low ordered phase embedded in an amorphous matrix [36] and this behaviour is further enhanced for poly(ethylenenaphthalate), PEN, copolymers and blends [37]. Control of the morphology developed can be made with a method similar to the one used in metallurgy for CCT, although a revision of the thermal conditions has helped to define its limits and advantages[38]. A significant advantage is the possibility to obtain, although in limited size, homogeneous samples. In this situation, the macroscopic characterization techniques are also representative of the utilized solidification conditions. Furthermore, the solidification conditions used in this technique allow to explore and simulate conditions that are normally encountered in polymer processing.

Important information on the processability and structure-property relations for PLSN can be gathered from the understanding of their rheological properties. In a broader view, linear and non-linear viscoelastic properties can be related to their nanoscale and mesoscopic structures [39]. For example, rheological properties of dispersions of layered silicates in low molecular weight solvents display negative thixotropy attributed to the breakdown and slow reformation of superstructures due to the application and subsequent removal of the shear [39]. In the case of polymer-layered silicates nanocomposites, melt rheological properties are related to the strength of interaction between polymer and silicate as well as to the viscoelastic properties of the macromolecular matrix, which, in turn, determine the mesoscopic structure of the nanolayers.
Dynamic-mechanical characterization performed on intercalated nanocomposites with a polystyrene-polyisoprene diblock copolymer showed that both storage and loss shear modulus increase with increasing silicate loading. The behavior at high frequencies (macromolecules have no time to fully relax) is qualitatively unaffected by the presence of nanolayers. As the frequency decreases the materials exhibit a diminished frequency dependence, which becomes weaker with increasing silicate content. This pseudo-solid-like behavior sets in above a critical silicate content [40].
Shear alignment of nanocomposites (e.g. y prolonged application of large-amplitude oscillatory shear) determine dramatic changes in the linear viscoelastic behavior: in fact both shear storage and loss moduli for the aligned nanocomposites are considerably lower than those for the initially unaligned samples. Moreover the frequency dependence of aligned samples is more pronounced, also at low frequencies, witnessing a breakdown of the percolated silicate network in the shear-aligned material.
Non-linear complex viscosity probed as function of strain amplitude at fixed frequency in intercalated nanocomposites show a sharp decrease in the complex viscosity with strain amplitude, consistently with the quiescent state mesoscopic structure which is disrupted by applying a shear deformation rate above a certain value.
The steady shear response of layered silicate nanocomposites is very important in view of the potential processability of these materials. Addition of small amounts of silicate results in an enhancement of viscosity. At higher silicate content, low shear rate viscosity displays a pseudo-solid-like behavior (presence of finite yield stress) while at high shear rates the shear thinning of the nanocomposites becomes similar to that of unfilled polymer, for all the nanofiller amounts (below 10 %). <<<