Linear viscoelastic behavior of poly(ethylene terephtalate) above Tg amorphous viscoelastic properties Vs crystallinity: Experimental and micromechanical modeling

Linear viscoelastic behavior of amorphous and semicrystalline poly(ethylene terephtalate), (PET), was experimentally investigated. PET’s samples with different crystallinities (Xc) were prepared and viscoelastically characterized. Based on our experimental results (properties of the amorphous PET and semicrystalline polymers), micromechanical model was used to, first predict the viscoelastic properties of the semicrystalline polymers and second predict the changes on the viscoelastic properties of the amorphous phase when the crystallinity increases. For the micromechanical modeling of semicrystalline material’s viscoelastic properties, difficulties lie on the used numerical methods (Laplace-Carson transformation) and also on the actual physical and mechanical properties of the amorphous phase. In this paper we tried to simplify the Laplace-Carson-based method by using a pseudo-elastic one that avoids the numerical difficulties encountered before. The time-dependant problem is so replaced by a frequency-dependant set of elastic equations. Good agreement with low crystallinity fraction was found however large discrepancies appear for medium and high crystallinity. The poor agreement raises the issue of which amorphous mechanical properties should be taken as input in the micromechanical model? According to the dynamic mechanical analysis (DMA) experimental data, multiple amorphous phases with different glass transition temperatures were observed for each tested semicrystalline sample. For each sample, new glass transition temperature related to an equivalent amorphous phase was determined. DMA tests done at 1 Hz help estimating the mechanical properties of the new amorphous phase based on its new glass transition temperature. Using the new micromechanical approach developed in this paper, the changes occurring on the viscoelastic behavior of the amorphous phase upon crystallization were estimated. Good agreement was found after comparing the micromechanically estimated amorphous behavior with the experimentally estimated one leading to believe that the physical and mechanical properties of the amorphous phase change upon crystallization and taking on account this phenomenon is a key to a good prediction of the semicrystalline behavior using micromechanical models.     

Studies on the thermal and mechanical behavior of PLA‐PET blends

The increasing use of bio‐sourced and biodegradable polymers such as poly(lactic acid) (PLA) in bottle packaging presents an increasing challenge to the polyethylene terephthalate (PET) recycling process. Despite advanced separation technologies to remove PLA from PET recyclate, PLA may still be found in rPET process streams. This study explores the effects of PLA on the mechanical properties and crystallization behavior of blends of PET containing 0.5–20% PLA produced by injection molding. SEM indicates an immiscible blend of the two polymers and TGA confirms the independent behavior of the two polymers under thermal degradation conditions. Temperature‐modulated DSC studies indicate that adding PLA to PET increases the rigid amorphous fraction of the PET moiety. Critical amounts of PLA induce stress oscillation behavior during mechanical testing. The mechanical behavior of the samples is explained by antagonistic interaction between increased rigid amorphous fraction and decreased fracture strength arising from an increased population of PLA microparticles. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 44147.     

The relationship between microstructure and toughness of biaxially oriented semicrystalline polyester films

The relationship between microstructure and toughness of biaxially stretched semicrystalline polyester films was investigated. Optically transparent films were prepared by simultaneous biaxial stretching of melt-cast sheets near the glass transition temperature. Copolyesters of polyethylene terephthalate (PET) with different compositions of two diols: ethylene glycol (EG) and cyclohexane dimethanol (CHDM), and stoichiometrically matched terephthalic acid were used to produce films with different degrees of crystallinity. In addition, the PET films with different crystalline morphologies were produced by constrained high temperature annealing of biaxially oriented films. The toughness, degree of crystallinity and crystalline morphology/molecular ordering were studied using mechanical testing, synchrotron small-angle X-ray scattering (SAXS), wide-angle X-ray diffraction (WAXD) techniques, and differential scanning calorimetry (DSC). The results indicate that the toughness of a semicrystalline polymeric film is determined by the interconnectivity of the crystalline phase within the amorphous phase and is greatly influenced by the degree of crystallinity and the underlying crystalline morphology.     

Dielectric and dynamic mechanical relaxation behaviour of poly(ethylene 2,6-naphthalene dicarboxylate). II. Semicrystalline oriented films

The dielectric and dynamic mechanical behaviour of bi-stretched non-treated and annealed semicrystalline poly(ethylene 2,6-naphthalene dicarboxylate) (PEN) films are studied as a function of different morphologies obtained by thermal treatments at temperatures close to the melting temperature of a semicrystalline film. Differential scanning calorimetry (DSC) shows that the glass transition temperatures do not change significantly with the thermal treatment for bi-stretched films. However, the melting temperatures and the degree of crystallinity increase with the value of annealing temperature. Both dielectric relaxation spectroscopy (DRS) and dynamic mechanical analysis (DMA) display three relaxation processes. In order of decreasing temperature, can be observed: the α-relaxation due to the glass transition, the β^∗-process assigned to cooperative molecular motions of the naphthalene groups which aggregate and the β-relaxation due to local fluctuations of the carbonyl groups. The α-relaxation process shifts to higher temperatures for the 250 and 260 °C treated bi-stretched semicrystalline samples compared to the sample thermally treated at 240 °C according to DRS data but shifts to lower temperatures according to the DMA measurements for the three annealed samples. This discrepency results from the different sensitivity of each methods with regards to the release of orientation. At a fixed frequency the temperature associated to β^∗-relaxation is lower for the non-treated bi-stretched semicrystalline samples than for the treated ones using DMA but no difference can be seen in DRS. The associated apparent activation energies are rather high which suggest cooperative motions. It is assumed that the orientation of the samples prevents coupling between the naphthalene groups due to the stretched chain configuration in the amorphous phase. The activation energy for the β-process given by DRS is independent of the thermal treatment and the value agrees with those found for poly(ethylene terephthalate) (PET) and amorphous PEN. Evidence of the decrease of orientation in the sample with thermal treatment can be seen via the onset of mobility, both by DRS and DMA. Thus, the orientation induces a greater change of properties compared to the crystalline samples obtained from the thermal treatment of an amorphous sample. Finally, a three phase model is proposed since there is evidence of a rigid amorphous phase present in PEN biaxially stretched samples which was favoured by the dependence of dielectric relaxation strengths on the degree of crystallinity for the β^∗- and α-relaxation.     

Mobile amorphous phase fragility in semi-crystalline polymers: Comparison of PET and PLLA

PET and PLLA were cold crystallised at various times and the two polymers were studied by differential scanning calorimetry (DSC), dielectric spectroscopy (DS) and thermally stimulated depolarisation currents (TSDC). The crystalline, the amorphous and the rigid amorphous fraction were quantified. The percentage of rigid amorphous fraction is very large in semi-crystalline PET and very low in semi-crystalline PLLA. From DSC, DS and TSDC data, the values of the relaxation times of four samples were obtained above and below the glass transition. The “strong-fragile” glass former liquid concept was used and the fragility of polymers was obtained. The presence of the crystalline phase and of a rigid amorphous fraction does not significantly modify PLLA fragility parameters and the polymer remains “fragile”, while for PET the semi-crystalline material goes towards a “strong character”. The coupling between phases is much weaker in PLLA than in PET.     

Films of PLLA/PHBV: Thermal,morphological, and mechanical characterization

Blends of poly(L ‐lactic acid)/poly(3‐hydroxybutyrate‐co‐3‐hydroxyvalerate) (PLLA/PHBV), both semicrystalline polymers, were prepared in different compositions (100/0, 80/20, 60/40, 50/50, 40/60, 20/80, and 0/100) and characterized by thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), dynamic mechanical analysis (DMA), scanning electron microscopy (SEM), polarized light microscopy (PLM), and tensile tests. Although PLLA/PHBV blends do not present clear phase separation by SEM, the analyses by TGA, DSC, and DMA showed that the PLLA/PHBV blends are immiscible. The cross sections observed by SEM showed that the morphology of the blends changes from porous to dense, due to the composition. DSC and DMA data showed two distinct glass transition and melting temperatures. However, the DMA analysis related to frequency variation showed partial molecular interactions between PHBV and PLLA. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 86: 2898–2906, 2002     

Compatible and crystallization properties of poly(lactic acid)/poly(butylene adipate‐co‐terephthalate) blends

Differential scanning calorimetry (DSC), wide angle X‐ray diffraction (WAXD) and dynamic mechanical analysis (DMA) properties of poly(lactic acid)/ poly(butylene adipate‐co‐terephthalate) (PLA/PBAT) specimens suggest that only small amounts of poor PLA and/or PBAT crystals are present in their corresponding melt crystallized specimens. In fact, the percentage crystallinity, peak melting temperature and onset re‐crystallization temperature values of PLA/PBAT specimens reduce gradually as their PBAT contents increase. However, the glass transition temperatures of PLA molecules found by DSC and DMA analysis reduce to the minimum value as the PBAT contents of PLA_xPBAT_y specimens reach 2.5 wt %. Further morphological and DMA analysis of PLA/PBAT specimens reveal that PBAT molecules are miscible with PLA molecules at PBAT contents equal to or less than 2.5 wt %, since no distinguished phase‐separated PBAT droplets and tan δ transitions were found on fracture surfaces and tan δ curves of PLA/PBAT specimens, respectively. In contrast to PLA, the PBAT specimen exhibits highly deformable properties. After blending proper amounts of PBAT in PLA, the inherent brittle deformation behavior of PLA was successfully improved. Possible reasons accounting for these interesting crystallization, compatible and tensile properties of PLA/PBAT specimens are proposed. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

The morphology,properties, and shape memory behavior of polylactic acid/thermoplastic polyurethane blends

Biodegradable polylactic acid (PLA) was compounded with thermoplastic polyurethane (TPU) by twin‐screw extrusion at weight ratios of 90/10, 80/20, 70/30, and 60/40. The blends were investigated based on their phase morphology, thermal and mechanical properties, and shape memory properties. The tensile results showed that PLA was successfully toughened by TPU. When the TPU content was 40%, the elongation‐at‐break increased to 400%. The SEM morphology showed that TPU was dispersed uniformly in the PLA matrix; DMA and DSC results indicated that the two polymers were immiscible. Most interestingly, it was found that the blends exhibited a shape memory behavior and, unlike most of the existing shape memory polymers (SMPs), the PLA/TPU blends could be deformed at room temperature without an extra heating and cooling step. During the deformation process, TPU acted as a toughening agent that prevented the PLA/TPU blends from breaking; thus, the temporary shape could be kept and internal stress was stored in the blends. Upon heating to above the glass transition temperature of PLA (about 60°C), the deformed parts regained their original shapes quickly along with the release of the stress. POLYM. ENG. SCI., 55:70–80, 2015. © 2014 Society of Plastics Engineers     

Evidence of two mobile amorphous phases in semicrystalline polylactide observed from calorimetric investigations

Amorphous phase dynamics in Poly(lactic acid) (PLA) with different crystallinity degrees have been investigated from the vitreous state to the glass transition by means of two calorimetric methods. Temperature Modulated Differential Scanning Calorimetry was used to characterize the heat capacity signals and the average cooperativity length at the glass transition in non‐aged materials. Standard DSC was used to study the physical aging. It is shown that amorphous and fully crystallized PLA exhibit different relaxation parameters. For semicrystalline PLA with an intermediate degree of crystallinity, the peaks of the enthalpy of recovery and the out‐of‐phase heat capacity component are bimodal. The bimodality of the peaks is attributed to the relaxations of the inter‐spherulitic and intra‐spherulitic amorphous phases, respectively. Thus, in partially crystallized PLA, the non‐crystalline fraction of the material could be divided in three fractions, namely the Rigid Amorphous Fraction, the inter‐spherulitic Mobile Amorphous Phase (MAP), and the intra‐spherulitic MAP. Each of them exhibits a distinct molecular mobility. POLYM. ENG. SCI., 54:1144–1150, 2014. © 2013 Society of Plastics Engineers     

Effect of organoclay in an immiscible poly(ethylene terephtalate) waste/poly(methyl methacrylate) blend

Blends of organically modified montmorillonite (OMMT) with poly(ethylene terephtalate) (PET) waste and poly(methyl methacrylate) (PMMA) were prepared by melt mixing. The morphology of PET/PMMA nanocomposites with different OMMT contents was characterized by transmission electron microscopy (TEM) and X‐ray diffraction (XRD). The nonisothermal crystallization temperatures of nanocomposites were also examined by DSC. TEM observations and XRD patterns revealed that silicate layers were intercalated and well dispersed in the blend. Nanocomposites displayed better mechanical properties when compared with the unfilled blend. DMA analyses also showed efficient mixing of the two immiscible polymers and changes in glass transition temperature with the presence of OMMT. DSC analysis showed an enhancement in crystallization rate of nanocomposites and a decrease in cristallinity. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Phase behavior and morphology in blends of poly(L‐lactic acid) and poly(butylene succinate)

Blends of poly(L ‐lactic acid) (PLA) and poly(butylene succinate) (PBS) were prepared with various compositions by a melt‐mixing method and the phase behavior, miscibility, and morphology were investigated using differential scanning calorimetry, wide‐angle X‐ray diffraction, small‐angle X‐ray scattering techniques, and polarized optical microscopy. The blend system exhibited a single glass transition over the entire composition range and its temperature decreased with an increasing weight fraction of the PBS component, but this depression was not significantly large. The DSC thermograms showed two distinct melting peaks over the entire composition range, indicating that these materials was classified as semicrystalline/semicrystalline blends. A depression of the equilibrium melting point of the PLA component was observed and the interaction parameter between PLA and PBS showed a negative value of ?0.15, which was derived using the Flory–Huggins equation. Small‐angle X‐ray scattering revealed that, in the blend system, the PBS component was expelled out of the interlamellar regions of PLA, which led to a significant decrease of a long‐period, amorphous layer thickness of PLA. For more than a 40% PBS content, significant crystallization‐induced phase separation was observed by polarized optical microscopy. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 86: 647–655, 2002     

The effect of poly(butylene succinate) content on the structure and thermal and mechanical properties of its blends with polylactide

Poly(butylene succinate) (PBS) and polylactide (PLA) were blended in a co‐rotating twin‐screw extruder with various contents of PBS from 0 to 100 wt%. The effect of PBS content on the thermal and mechanical properties of PBS/PLA blends was investigated by using DSC, softening point measurements, a Charpy impact test and tensile testing. The Fourier transform infrared spectra showed that the polymers are immiscible, but the addition of PBS could modify the PLA structure in PBS/PLA blends by changing the content of amorphous and crystalline phases. In addition, the cold crystallization temperature of PLA in blends decreases in comparison with pure PLA, which shows that PBS could have a plasticizing effect on PLA. This is confirmed by the results of DSC analysis. The mechanical properties of the blends depend on the percentage of PBS addition. Typically, the mechanical properties of PBS/PLA blends are intermediate between the properties of the polyesters from which they are obtained. However, in some cases unexpected changes in mechanical properties of the blends were observed. For example, the elongation at break for a PBS/PLA blend containing 10 wt% PLA is higher than for pure PBS. © 2019 Society of Chemical Industry     

Solvent‐ and thermal‐induced crystallization of poly‐L‐lactic acid in supercritical CO2 medium

The effect of different annealing treatments with supercritical carbon dioxide (SCCO₂) on the structural and mechanical properties of semicrystalline poly‐L ‐lactic acid (L ‐PLA) was investigated. 2000, 27,000, 100,000, and 350,000 g mol^?1 molecular weight L ‐PLA polymers were used in the study. The solid‐state processing of L ‐PLA at temperatures lower than the effective melting point led to solvent‐ and thermal‐induced crystallization. Solvent‐induced and isothermal crystallization mechanisms could be considered similar regarding the increase of polymer chain mobility and mass‐transfer in the amorphous region; however, quite different microstructures were obtained. SCCO₂ solvent‐induced crystallization led to polymers with high crystallinity and melting point. On the contrary, SCCO₂ thermal‐induced crystallization led to polymers with high crystallinity and low melting point. For these polymers, the hardness increased and the elasticity decreased. Finally, the effect of dissolving SCCO₂ in the molten polymer (cooling from the melt) was analyzed. Cooling from the melt led to polymers with high crystallinity, low melting point, low hardness, and low elasticity. Distinctive crystal growth and nucleation episodes were identified. This work also addressed the interaction of SCCO₂‐drug (triflusal) solution with semicrystalline L ‐PLA. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Peculiar crystallization kinetics of biodegradable poly(lactic acid)/poly(propylene carbonate) blends

Binary blends of poly(lactic acid) (PLA) and poly(propylene carbonate) (PPC) were found to display a peculiar crystallization kinetics. The two biodegradable polymers were blended by melt mixing, to obtain binary blends at various compositions. Temperature‐modulated calorimetry and dynamic‐mechanical analysis indicated that the blend components are partially miscible, and display two separate glass transitions, at temperatures intermediate to those of the plain polymers. Electron microscopy analysis disclosed the morphology of PLA/PPC blends, made of PPC‐rich particles finely dispersed within the PLA‐rich matrix. The possible establishment of interactions between the functional groups of the two polymers upon melt mixing has been hypothesized as the reason for partial miscibility and compatibility of the two biodegradable polymers. The PLA/PPC blends display good mechanical properties, with enhanced performance at rupture compared with plain PLA. Most importantly, the addition of PPC affects also the crystallization kinetics of PLA, since the more mobile PPC chains favor diffusion of the stiffer PLA chain segments towards the growing crystals, which fastens the spherulite growth rate of PLA. Such positive influence of an amorphous polymer on crystal growth rate has been demonstrated here for the first time in blends that display phase‐separation in the melt. POLYM. ENG. SCI., 55:2698–2705, 2015. © 2015 Society of Plastics Engineers     

Influence of different glass fiber sizings on selected mechanical properties of PET/glass composites

The properties of fiber-reinforced plastics are considerably influenced by fiber-matrix interaction. The aim of this study was to investigate the influence of glass fiber surface treatments on the morphology of poly(ethylene terephthalate) (PET) and on selected mechanical properties of unidirectional PET/glass fiber composites. The materials used here were E-glass fibers treated with model sizings including aminosilane as a coupling agent and polyurethane and epoxy resin dispersions as film formers and PET as the matrix. For identification of the degree of crystallinity of the PET matrix, differential scanning calorimetry (DSC) was used. To study the influence of the different sizings on the mechanical properties, the following tests were performed: interlaminar and intralaminar shear tests and a transverse tensile test. Dynamic-mechanical analysis (DMA) was used to characterize the behavior of the composites under dynamical load. The DSC results show that the overall crystallinity and the melting behavior of the PET matrix were hardly influenced by the glass fiber surface treatments used. The various strength properties of the composites are influenced not only by the silane coupling agent, but also by the type of film former. With an epoxy resin dispersion, the mechanical properties were enhanced compared with a polyurethane dispersion. These results were confirmed by characterization of the composites by DMA.     

Synthesis and Properties of some ε‐Caprolactone‐Based Di‐ and Triblock Polymers by Anionic Polymerization

Block copolymers containing ε‐caprolactone were synthesized. Mechanical properties as a function of chemical composition and domain structure as a function of elongation were studied. Based on previous optimal conditions determination by factorial design of experiments of ε‐caprolactone anionic polymerization, polystyrene‐block‐poly(ε‐caprolactone), polyisoprene‐block‐poly(ε‐caprolactone), polystyrene‐block‐polybutadiene‐block‐poly(ε‐caprolactone) (SBCL), and polystyrene‐block‐polyisoprene‐block‐poly(ε‐caprolactone) (SICL) with different compositions where synthesized, and characterized by GPC and DSC. Both the SICL and SBCL materials are thermoplastic elastomers, from which spin‐cast films were prepared. Their mechanical properties were determined, small angle X‐ray scattering (SAXS) measurements were carried out during straining, and dynamic mechanical analysis (DMA) was performed. All diblock polymers separate into a two‐phase structure, but the melting point of crystalline poly(ε‐caprolactone) domains in the block polymer is higher than in the case of the homopolymer. According to DMA data, some of the SICL and SBCL materials are three‐phase systems, but others are only two‐phase systems. The two‐phase materials show a considerable depression of the composite hard domain glass transition and, consequently, turn out to be very soft. It appears peculiar that the transition from three‐phase to two‐phase material is accomplished by decreasing the soft block length. For the soft material SAXS exhibits a lamellar stack nanoscale structure and several reflections of colloidal crystals. As a function of increasing elongation, the crystal reflections broaden, whereas lamellar stacks rotate as a whole.     

Effects of annealing on relaxation behavior and charge trapping in film-processed poly(phenylene sulfide)

The relaxation behavior of poly(phenylene sulfide) (PPS) Ryton^TM film has been studied as a function of annealing temperatures, T_a, ranging from 30°C to 140°C. Previously, this type of semicrystalline PPS film was shown to possess a very large fraction of constrained, or rigid, amorphous chains. Here we investigate relaxation of amorphous chains using differential scanning calorimetry (DSC), dynamic mechanical analysis (DMA), and thermally stimulated depolarization current (TSDC). DSC studies suggest that annealing causes the as-received PPS film to relax some of its rigid amorphous fraction and increase its crystallinity, for T_a > T_g. DMA results show a corresponding increase in the temperature location of the dissipation peak and a decrease in its amplitude when T_a increases above 100°C. Analysis of the TSDC p-peak due to injected space charges trapped at the crystal/amorphous interphase provides additional information about amorphous phase relaxation. This peak does not exist in amorphous film, or as-received film, or as-received films annealed at lower T_a. The p-peak does exist in cold-crystallized films and as-received films annealed at higher T_a. We suggest that crystallinity is a necessary, but not sufficient, condition for observation of a p-peak. In addition to crystallinity, the sufficient conditions for observation of a p-peak are existence of (1) a sharp and distinct crystal/amorphous interphase, to provide charge trapping, and, (2) a large fraction of liquid-like amorphous phase, to provide a pathway for charge transport. These conditions are not met in PPS semicrystalline films with very imperfect crystals and large amounts of rigid amorphous phase. In such films, the rigid amorphous phase has, like the crystal phase, a restricted molecular mobility that causes it also to restrict the mobility of space charge. The implications are that film PPS processed with a large amount of rigid amorphous phase chains will have superiour barrier properties to the build-up of interfacial space charge. © 1996 John Wiley & Sons, Inc.     

Surface modification of spruce wood flour and effects on the dynamic fragility of PLA/wood composites

Poly (lactic acid) (PLA), a biodegradable aliphatic semicrystalline polyester was filled with 40 wt% spruce wood flour (WF), to produce composite materials. Hydrothermal treatment, as well as maleic anhydride, vinyltrimethoxysilane, and stearic acid surface treatments were applied. The influence of surface modifications for WF was tested in terms of thermal, mechanical, and viscoelastic properties. The recorded results show that in both, the untreated and treated PLA/WF composites, the rigid amorphous phase content has been enhanced. The presence of WF causes a stiffness increase of the PLA/WF composites, while damping factor was decreased. The effect of wood surface modifications on interfacial compatibility with PLA was estimated by dynamic fragility parameter m calculated according the Williams‐Landel‐Ferry equation. The incorporation of untreated WF increased dynamic fragility of PLA/WF composites markedly, whereas used silane, maleic anhydride and hydrothermal treatments lead to lower values of parameter m. POLYM. ENG. SCI., 2011. © 2010 Society of Plastics Engineers     

An in-depth study on crystallization kinetics of PET/PLA blends

The crystallization behavior and mechanical properties of PET/PLA blends with various amounts of PLA were investigated using a wide angle X-ray diffraction (WAXD), differential scanning calorimeter (DSC) and tensile analyses. The crystallization rate and relative crystallinity of the PET/PLA blends were studied by theoretical models of Kissinger, Avrami, Ziabicki and Ozawa. The WAXD analysis showed that the PLA phase was wholly amorphous in all blends after cooling from the melt to ambient temperature. Crystallization behavior assessments on PET/PLA blends suggest that PLA acts as a nucleating agent for PET phase leading to an increase in the initial and peak crystallization temperatures. Kissinger’s model showed a rise in activation energy up to 72% for the PET/PLA blends containing 30 wt% PLA. Ziabicki’s model gave a minimum value for kinetic parameter in PET/PLA (70/30 w/w) due to the nucleating action of PLA. On the other hand, PLA acted as a retarder for chain segments of PET tending to diffuse through the surface of growing crystals. Therefore, at an optimal composition of PET/PLA, crystallization occurs appropriately. However, an increase in PET content leads to fall in ductility, tensile strength, modulus, elongation-at-break, and fracture toughness of PET/PLA blends.     

Glass Transition Temperature and the Nature of the Amorphous Phase in Semicrystalline Polymers: Effects of Drawing,Annealing and Hydration in Polyamide 6

Abstract

The influence of the organization of the amorphous chains segments on the glass transition temperature (T_g) in semicrystalline polymers is analysed by studying the effects of drawing, annealing and hydration in polyamide 6 fibers. We consider the role of three of the features of the amorphous phase: orientation (configurational entropy), density (free volume) and confinement (segmental mobility). Three classes of amorphous phases are identified; two of these are constrained in the intercrystaline regions, at the fold surfaces (between the lamellae within the lamellar stack) and at the stem surface (growth surface of the lamellae or between the fibrils). The third species is the bulk amorphous phase outside the lamellar stacks, and constitutes a large fraction of the amorphous phase especially at low crystallinities. Because the small fraction of the amorphous chain segments in the intercrystalline regions, and because they are in confined spaces, we suggest that these interlamellar and the interfibrillar components do not contribute significantly to the observed major glass transition peak. Rather, it is the amorphous region outside the lamellar stack that determines the T_g. T_g increases upon drawing and decreases upon annealing (heat setting). Our data suggest that orientation has a direct influence on T_g and can easily be measured whereas the influence of crystallinity is more complex. The influence of orientation of T_g can be understood in terms of a two T_g model in which the oriented amorphous component has a higher T_g than the unoriented component.