Dynamic mechanical and dielectric relaxation characteristics of poly(trimethylene terephthalate)

The dynamic mechanical and dielectric relaxation properties of a commercial poly(trimethylene terephthalate) [PTT] have been investigated for both quenched and isothermally melt-crystallized specimen films. The relaxation characteristics of PTT were consistent with those of other low-crystallinity semiflexible polymers, e.g. PET and PEEK. While the sub-glass relaxation was largely unperturbed by the presence of the crystalline phase, both calorimetric and broadband dielectric measurements across the glass transition indicated the existence of a sizeable rigid amorphous phase (RAP) fraction in melt-crystallized PTT owing to the constraining influence of the crystal surfaces over the crystal-amorphous interphase region. A strong increase in measured dielectric relaxation intensity (Δ?) with temperature above T_g indicated the progressive mobilization of the RAP material, as well as an overall loss of correlation amongst the responding dipoles.     

TSC study of length ester side group effect on sub-vitreous relaxations in poly(n-alkyl methacrylates)

A comparative study by thermo-stimulated currents (TSC) of three poly(n-alkyl methacrylates) is performed. TSC complex spectra exhibit two dipolar relaxations considered in the following: the glass relaxation and a secondary relaxation related to motion of ester side groups. For each sample, the fractional polarisations protocol is applied to describe the fine structure of these complex spectra. It allows us to define the evolution of activation parameters with temperature. The influence of side chain architecture is then considered. It emphasises that the β mode is independent of the ester group length. On the contrary, the α mode is highly sensitive to structural change. These results are finally explained in terms of molecular mobility.     

Molecular relaxation in cross-linked poly(ethylene glycol) and poly(propylene glycol) diacrylate networks by dielectric spectroscopy

The molecular relaxation characteristics of rubbery amorphous crosslinked networks based on poly(ethylene glycol) diacrylate [PEGDA] and poly(propylene glycol) diacrylate [PPGDA] have been investigated using broadband dielectric spectroscopy. Dielectric spectra measured across the sub-glass transition region indicate the emergence of an intermediate “fast” relaxation in the highly crosslinked networks that appears to correspond to a subset of segmental motions that are more local and less cooperative as compared to those associated with the glass transition. This process, which is similar to a distinct sub-T_g relaxation detected in poly(ethylene oxide) [PEO], may be a general feature in systems with a sufficient level of chemical or physical constraint, as it is observed in the crosslinked networks, crystalline PEO, and PEO-based nanocomposites.     

New comb-like poly(n-alkyl itaconate)s with crystalizable side chains

A series of poly(mono n-alkyl itaconate)s, poly(methyl n-alkyl itaconate)s and poly(di n-alkyl itaconate)s with n=12, 14, 16, 18 and 22 have been prepared by radical polymerization. NMR studies point out that poly(methyl n-alkyl itaconate)s and poly(di n-alkyl itaconate)s are mainly syndiotactic polymers whereas poly(mono n-alkyl itaconate)s are obtained as almost atactic polymers. The characterization performed by DSC, solid state ¹³C CP/MAS NMR and X-ray diffraction, indicates that the side chains of poly(mono n-alkyl itaconate)s and poly(methyl n-alkyl itaconate)s derivatives with more than 12 carbon atoms are able to crystallize in hexagonal lattices. In the case of poly(di n-alkyl itaconate)s, when the side chains contain 12 or more carbon atoms, they are able to crystallize also in hexagonal lattices.     

Dynamic relaxation characteristics of crosslinked poly(ethylene oxide) copolymer networks: Influence of short chain pendant groups

The dynamic relaxation characteristics of short-branch rubbery amorphous networks prepared by the photopolymerization of poly(ethylene glycol) diacrylate [PEGDA] crosslinker have been investigated using dynamic mechanical analysis and broadband dielectric spectroscopy. Copolymerization with low molecular weight acrylates was used to control effective crosslink density in the networks and led to the insertion of ethylene oxide pendant groups along the network backbone. Substantial differences in the sub-glass and glass-rubber relaxation properties of the copolymers were observed as a function of pendant length and the nature of the pendant terminal group (e.g., -OH vs. -OCH₃); the results are compared with prior studies on model copolymers containing longer, more flexible side branches.     

Relaxation dynamics and cold crystallization of poly(pentamethylene terephthalate) as revealed by dielectric spectroscopy

The relaxation dynamics of poly(pentamethylene terephthalate) has been investigated by means of dielectric spectroscopy. The sub-glass dynamics is characterized by the existence of a bimodal β process whose faster and slower components have been assigned to the relaxation of the bond between the ester oxygen and the aliphatic carbon and to the link between the aromatic ring carbon and the ester carbon, respectively. By comparison with other closely related aromatic polyesters it is shown that the faster component strongly depends on the amount of methylene groups while the slower one is not considerably affected by the nature of the glycol subunit. The changes in the α process associated to the segmental relaxation during cold crystallization reveal the formation of a rigid amorphous phase fraction. Combination of dielectric experiments with X-ray scattering ones suggests that during cold crystallization PPT crystal lamellae tend to fill the space homogeneously.     

Unusual accelerated molecular relaxations of a tin fluorophosphate glass/polyamide 6 hybrid studied by broadband dielectric spectroscopy

Phosphate glass (Pglass)/polymer hybrids are a relatively new class of materials that combine the advantages of classical polymer blends and composites without their disadvantages. In the case of highly interacting Pglass/polymer (i.e., polyamide 6) hybrids, counter-intuitive properties that are difficult to explain are often observed. To shed light into the origins of the special behavior of the hybrids, we investigated the molecular relaxation processes in the hybrids using broadband dielectric spectroscopy. The dielectric loss spectra were fitted with the Havriliak-Negami equation and the characteristic relaxation times of the hybrid and the pure components were observed. The temperature dependence of the characteristic relaxation times was described using either the Vogel-Fulcher-Tammann, for the α-relaxations, or an Arrhenius type equation, for the β- and γ-relaxations. The addition of Pglass greatly accelerated both the α- and β-relaxations of the polyamide 6. However, the γ-relaxation was found to be independent of Pglass composition. This suggests partial miscibility in the solid state, which was confirmed via NMR spectroscopy. The unexpected dramatic change in the β-relaxation process in the 10 vol.% Pglass hybrid suggests that blending can change the local environment of polyamide 6 due to the nanoscale morphology of this system as confirmed by TEM and NMR. It is thought that the fraction of miscible Pglass disrupts the hydrogen bonding between polyamide 6 chains and thereby reduces coordinated, multiple chain motion. In turn, this produces a plasticization effect and possible modification of the polyamide 6's crystalline structure in the Pglass/polyamide 6 hybrids.     

Dynamic mechanical and tensile properties of poly(N-vinyl pyrrolidone)-poly(ethylene glycol) blends

Mechanical properties of miscible blends of high molecular weight poly(N-vinyl pyrrolidone) (PVP) with a short-chain, liquid poly(ethylene glycol) (PEG) of molecular weight 400 g/mol have been examined as a function of PVP-PEG composition and degree of hydration. The small-strain behavior in the linear elastic region has been evaluated with the dynamic mechanical analysis and compared with the viscoelastic behavior of PVP-PEG blends under large strains in the course of uniaxial drawing to fracture and under cyclic extension. A strong decoupling between the small-strain and the large strain properties of the blends has been observed, indicative of a pronounced deviation from rubber elasticity in the behavior of the blends. This deviation, also seen on tensile tests under fast drawing, is attributed to the peculiar phase behavior of the blends and the molecular mechanism of PVP-PEG interaction. Nevertheless, for the PVP blend with 36% PEG, under comparatively low extension rates, the reversible contribution to the total work of deformation up to ε=300% has been found to be maximum at around 70%, while the blends containing 31 and 41% PEG behave rather as an elastic-plastic solid and a viscoelastic liquid, respectively.     

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.     

Dynamic mechanical and melt rheological properties of sulfonated poly(butylene succinate) ionomers

A series of poly(butylene succinate) ionomers (PBSi) containing 5-sodium sulfoisophthalate units were prepared by bulk polymerization of succinic acid and 1,4-butanediol in the presence of dimethyl 5-sulfoisophthalate sodium salt (DMSI) up to 5 mol% of diacid monomer. Conspicuous variation of the storage modulus for PBSi was observed, depending on the content of DMSI. The increasing rate of cluster T_g was lower compared to those of amorphous polymer-based ionomers. These results were probably due to the lower clustering ability of semi-crystalline PBSi as compared with amorphous-based ionomers. Non-contact atomic force microscopy confirmed that the size of PBSi-3 clusters was about 40∼50 nm, demonstrating that the clusters were aggregated. Melt rheological analysis was carried out to investigate the effects of ionic groups on the rheological properties as a function of temperature or shear force in the molten state. The melt viscosities of PBSi showed higher values than the parent PBS up to about 190 °C, while with further increasing temperature a falling inflection region of melt viscosity was observed. It was suggested that the relaxation of PBSi chains was due to the thermal dissociation of ionic aggregates.     

Glass transition behaviour of poly(ether ether ketone)/poly(aryl ether sulphone) blends: dynamic mechanical and dielectric relaxation studies

Blends of poly(ether ether ketone) (PEEK) and poly(aryl ether sulphone) (PES) have been prepared in the whole composition range. The molecular dynamics and α-relaxation behaviour of these materials have been studied using dynamic mechanical and dielectric relaxation spectroscopy. From dynamic mechanical relaxation studies, two α-relaxation peaks corresponding to the segmental relaxation process of pure components in the blend was observed. Also, it was found that the temperature at which α-process of the homopolymers occurs, shows a slight change with blend composition, corresponding to a PEEK-rich and PES-rich phase. The relaxation intensities of the homopolymers in the blend compared to that in pure state were approximately proportional to their respective content in the blend. From the phase composition of the respective phases obtained using Fox equation, it has been inferred that PEEK dissolves more in PES than vice-versa. The α-relaxation of PES could not be detected from dielectric relaxation spectroscopy because of the possible influence of dc conduction and electrode polarization losses. Otherwise, the α-relaxation behaviour of PEEK-rich phase observed from dielectric relaxation studies agree with those inferred from dynamic mechanical relaxation studies. Furthermore, activation energies for molecular motions (E_a) at the α-relaxation have also been determined using an Arrhenius form of equation and it has been found that E_a for both PEEK-rich and PES-rich phase show variation with composition. Similarly, the relaxation times associated with the mobility of relaxing species in both PEEK and PES are influenced in the blends. It is likely that these observations are related to some interactions and a partial segmental mixing between the blend components, which result in changes in the local molecular environment on blending.     

Configurational statistics of poly(L-lactide) and poly(DL-lactide) chains

Conformational characteristics of poly(lactide)s have been investigated by density functional theory and ab initio molecular orbital (MO) calculations and NMR experiments on model compounds. Characteristic ratios, configurational entropies, and internal energies of poly(L-lactide) and poly(DL-lactide), whose stereosequences were generated by Bernoulli and Markov stochastic processes, were calculated under the refined rotational isomeric state scheme with conformational energies and geometrical parameters derived from the MO calculations. In terms of the conformational characteristics thus revealed, we have elucidated the reason why unperturbed chain dimensions determined experimentally for poly(L-lactide) are scattered considerably and, furthermore, discussed crystallization and crystal structures of poly(L-lactide) and molecular characteristics of poly(DL-lactide) synthesized from rac-lactide with stereospecific polymerization catalysts.     

Dielectric spectroscopy of poly(butylene succinate) films

Dielectric properties of poly(butylene succinate) crystallized under different conditions have been reported in the temperature range of 163-383 K and in the frequency range of 0.01-10⁵ Hz. Both the dipolar α and β processes have been identified at low temperatures: the α process is associated with the amorphous fraction while the β with the relaxations in both the amorphous and crystalline fractions. The space charge effect dominates the high temperature dielectric spectra. These spectra have been analyzed in the light of an equivalent circuit model. The Maxwell-Wagner-Sillars polarization, electrode polarization and free charge motion are well resolved. At 383 K, near the melting temperature (387 K), massive melting and subsequent recrystallization have been observed. The peculiar evolution of the spectra is also analyzed using the same equivalent circuit model. The relationship between the fitting parameters and the evolved microstructures is discussed.     

Calorimetric and dielectric study on poly(trimethylene terephthalate)/polycarbonate blends

Poly(trimethylene terephthalate) (PTT)/polycarbonate (PC) blends with different compositions were prepared by melt blending. The miscibility and phase behavior of melt-quenched and cold-crystallized blends were studied using differential scanning calorimetry (DSC) and dielectric relaxation spectroscopy. The blends of all compositions display only one glass transition (T _g ) in both states. The melting temperature and the crystallinity of PTT in the blend decrease with increasing PC content. The dielectric results for the melt-quenched blends, for PC content up to 60 wt.%, exhibited two merged relaxation peaks during the heating scan; the lower temperature relaxation peak represent the normal glass-transition (α) relaxation of the mixed amorphous phase and the higher temperature relaxation due to the new-constrained mixed amorphous phase after crystallization. Cold-crystallized blends displayed only one glass transition α-relaxation whose temperatures varied with composition in manner similar to that observed by DSC. The dielectric α-relaxation of cold crystallized blends has been analyzed. Parameters relating to relaxation broadening, dielectric relaxation strength, and activation energy were quantified and were found to be composition dependent. The PTT/PC blends could be considered as two-phase system, a crystalline PTT phase and a mixed amorphous phase consisting of a miscible mixture of the two polymers. However, the crystallinity was only detected for blends containing greater than 40 wt.% PTT.     

Molecular dynamics simulation studies of binary blend miscibility of poly(3-hydroxybutyrate) and poly(ethylene oxide)

By means of full atomistic molecular dynamics simulation, the solubility parameters for pure poly(3-hydroxybutyrate) and poly(ethylene oxide) are calculated and the results are in agreement with the literature values. Furthermore, in order to reveal the blend property, the volume-temperature curve of the PHB/PEO blend system (1:2 blends in terms of repeated units) is simulated by employing the united atom approximation to obtain the glass transition temperature. From the volume-temperature curve, the glass transition temperature is about 258 K, which is compared well with the experimental results. It should be pointed out that the two simulated solubility parameters are similar and there is only one glass transition of the blend system, these indicate that the studied blend system is miscible.     

Effects of CO2 treatments on the dual glassy relaxations and dual melting peaks in poly(ethylene terephthalate)

In this study, poly(ethylene terephthalate) (PET), an important packaging material for carbonated beverages, was investigated on its glassy relaxations and melt crystallizations in CO₂ in a high-pressure differential scanning calorimeter (PDSC) and a high-pressure thermostat chamber as a function of CO₂ pressure, time, CO₂ depressurization rate, and PET crystallinity. DSC measurements found a low glass transition temperature (T_gL) at near 50 °C in the wholly amorphous PET and a high glass transition temperature (T_gH) at near 70 °C in the PET sample with a fairly high crystallinity (X_c 50%). Both T_gL and T_gH decreased with increasing time in CO₂, attributed to plasticization by CO₂. PET sample with a moderate crystallinity (X_c 42%), however, exhibited both T_gL and T_gH corresponding to the relaxations of the dual amorphous phases as confirmed by dynamic mechanical analysis, with the T_gL assigned to the free amorphous phase and the T_gH to the constrained amorphous phase. The T_gL is much farther apart from T_gH than those obtained in N₂. The T_gL and T_gH in PET with a moderate crystallinity unexpectedly appeared to be insignificantly varied with time in CO₂; however, the magnitude of the T_gL signal increased but the T_gH signal decreased with increasing time in CO₂, attributed to disentanglement of polymer chains by CO₂. Dual melting peaks in PET were found after nonisothermal crystallization from the melt in CO₂. Temperature-modulated DSC (TMDSC) analysis indicated that melting-recrystallization model and double lamellar thickness model were both responsible for the appearance of the dual melting peaks.     

Type-A dipole moment and segmental dynamics of poly(styrene oxide)

We report the dielectric properties of poly(styrene oxide)s (PSO) in bulk and concentrated solution states. Since the structure of PSO is asymmetric along the backbone, the repeat unit of PSO is expected to possess the non-zero component of the dipole moment p_A parallel to the chain contour as well as the perpendicular component p_B. The former and latter cause the dielectric normal mode and segmental mode relaxations, respectively. Contrary to the above mentioned expectation the temperature dependence of ε″ exhibits only the primary (α) relaxation and weak secondary relaxation (β) in the glassy state. No loss peak due to the normal mode relaxation was observed in the frequency region expected from the viscoelastic terminal relaxation in bulk and toluene solutions. The dielectric behaviours of the α relaxation in the bulk state were analyzed in detail and the parameters of the Vogel-Fulcher and the Havriliak-Negami equations were determined. The Kirkwood correlation factor was determined to be 0.36. The ¹³C NMR spectra indicate that the present PSO samples contain about 2% head-to-head linkages. This cannot be the origin of the disappearance of the normal mode. We conclude that p_A of PSO is too small to be detected. The p_A calculated with molecular orbital methods supports this conclusion.     

Plasticization of semicrystalline poly(l-lactide) with poly(propylene glycol)

Plasticization of semicrystalline poly(l-lactide) (PLA) with a new plasticizer - poly(propylene glycol) (PPG) is described. PLA was plasticized with PPG with nominal M_w of 425 g/mol (PPG4) and 1000 g/mol (PPG1) and crystallized. The plasticization decreased T_g, which was reflected in a lower yield stress and improved elongation at break. The crystallization in the blends was accompanied by a phase separation facilitated by an increase of plasticizer concentration in the amorphous phase and by annealing of blends at crystallization temperature. The ultimate properties of the blends with high plasticizer contents correlated with the acceleration of spherulite growth rate that reflected accumulation of plasticizer in front of growing spherulites causing weakness of interspherulitic boundaries. In PLA/PPG1 blends the phase separation was the most intense leading to the formation of PPG1 droplets, which facilitated plastic deformation of the blends that enabled to achieve the elongation at break of about 90-100% for 10 and 12.5 wt% PPG1 content in spite of relatively high T_g of PLA rich phase of the respective blends, 46.1-47.6 °C. Poly(ethylene glycol) (PEG), long known as a plasticizer for PLA, with nominal M_w of 600 g/mol, was also used to plasticize PLA for comparison.     

Study of secondary relaxations of poly(ethylene terephthalate) by photoluminescence technique

The α and β relaxation processes in two types of poly(ethylene terephthalate) with different degrees of crystallinity were studied by means of three methods, differential scanning calorimetry, dynamic-mechanical analysis and fluorescence spectroscopy. Information provided is complementary in the mean that every method sense phenomena that may occur at different times and length scales. Several probes, Coumarin 152 (C152), Coumarin 153 (C153), Coumarin 337 (C337) and 4′-dimethylamino-4-nitrostilbene (DMANS), were adsorbed in polymer films, and their fluorescence analysed over the temperature range from −150 to 150 °C. In general, a decrease in fluorescence intensity of probes as temperature increase was observed. This behaviour has been explained as a consequence of the enhancement of the free volume fraction that favoured the radiationless process of the lowest excited singlet state. Plots of fluorescence intensity versus temperature showed changes around the secondary relaxation temperatures. Therefore, good correlations between fluorescence and dynamic mechanical and calorimetric analysis were established. The obtained results indicated that the fluorescence from the probes incorporated to the material was dependent on the crystallinity of polymer. It would indicate that the fluorescence emission from those probes can be used to analyse annealing processes in semicrystalline polymers.     

Electrorheological properties of poly(o-anisidine) and poly(o-anisidine)-coated silica suspensions

Poly(o-anisidine) (PANs) and PANs-coated silica particles were prepared by oxidative polymerization of o-anisidine using ammonium persulphate in the absence and presence of silica particles, respectively. The entire surface of the silica particles was completely covered with PANs polymerized using low concentration of o-anisidine. The effects on the electrorheological behavior of surface modification of the silica particles coated with PANs were studied under oscillatory shear. Electrorheological properties of PANs and PANs-coated silica particle suspensions were found to exhibit higher performance in nonaqueous solutions than those of silica particle suspensions even at high temperatures.