Glass Transition Zone Research Articles

Segmental relaxation in miscible polymer blends

Usually the shape of the glass transition dispersion in the mechanical or dielectric spectra of pure polymers is skewed toward higher frequencies. In miscible polymer blends not only is this peak broader than in pure polymers, the broadening is often asymmetric towards lower frequencies. Concentration fluctuations are the obvious source of the broadening; however, a simple distribution of relaxation times, corresponding to a distribution in local compositions, would not account for the reversal in the asymmetry of the dispersion. Miscible blends are also thermorheological complex, with the temperature dependence of segmental relaxation exhibiting idiosyncratic composition dependencies. A model to describe the composition dependence and shape of the relaxation spectra of miscible polymer blends in the glass transition zone is described. The fluctuations in local composition inherent to a miscible blend give rise to a distribution in both the relaxation time and degree of cooperativity of segmental relaxation. Generally, the intermolecular cooperativity will be amplified by a high relative abundance of the component of the blend with the higher glass transition temperature; at least for the blends studied herein, higher TG is associated with a stronger capacity for intermolecular coupling of the segments. At fixed blend composition this effect governs the shape of the dispersion, as well as being manifested in the composition dependence of the segmental relaxation time. Since the response at lower frequency reflects the contribution of segments residing in regions richer in the high TG component, it is anticipated that the glass transition dispersion in miscible blends will be asymmetrically broadened towards lower frequencies. Application of the model to some miscible blends, including poly(vinylethylene)/polyisoprene, polyvinylmethylether/polystyrene, and tetramethyl polycarbonate/polystyrene), is demonstrated to successfully describe their most prominent features.

Trends in the temperature dependency of segmental relaxation in tetramethylbisphenol A polycarbonate/polystyrene blends

A model to account for the composition dependence and shape of the relaxation spectra of miscible blends in the glass transition zone is applied to dielectric measurements on tetramethyLBiapheno1 A polycarbonate (TMPC) mixed with polystyrene (PS). The shape of the dispersion associated with segmental relaxation is governed primarily by the fluctuations in the degree of cooperativity associated with the distribution in local composition arising from concentration fluctuations. A blend of fixed composition possesses a distribution of local environments; these effect a distribution in the degree of cooperativity of the segmental relaxation of locally rearranging chain units. Pure PS has a lower TB than TMPC. Hence, in the blend before cooperative dynamics is considered, PS chains will tend to have segmentally relaxed when segmental relaxation of TMPC is considered. The dynamical constraints imposed by PS segments in a local environment of TMPC are thus mitigated, allowing TMPC segmental relaxation to proceed with a lower degree of intermolecular cooperativity than is the case in less PS-rich environments. The temperature dependence of the frequency- temperature shift factors for the blend are governed by the degree of intermolecular cooperativity of the segmental relaxation. Hence, there exists a direct and verifiable relation between frequency and temperature dependencies of the mixture. The present data on TMPCJPS, though not sufficient to entirely demonstrate such a relationship, are nevertheless consistent with the predicted relation.

When is a polymer a polymer? A light scattering study of crossover from viscous fluid-like behaviour to chain constrained dynamics

Light (Brillouin and Raman) scattering studies of a polymer (EO) n of increasing chain lenght give evidence of crossover at a length of about 25–30 backbone bonds from molecular weight M w dependence to M w independence in the dynamics related to the glass transition. The crossover gives an indication of the number of interacting particles and the size of the cooperatively rearranging regions in the polymer in the dynamic glass transition zone.

Characteristic length of glass transition: experimental evidence

The glass transition length scale determined from formulae of a fluctuation approach (2ξa = 2 ... 5 nm) is confirmed by two independent experimental methods. Firstly the glass transition parameters in self-organized amorphous layers of variable thickness in semicrystalline poly(ethylene terephtalate) show significant sensitivity to the nanometer length scale of ξa. The results can be explained by a direct influence of spatial limitations on the large mode lengths of the glass transition. Secondly, the analysis of the arrangement of different system responses across the glass transition zone at constant temperature in polystyrene and poly(vinyl acetate) results in the location of ξa between the limits 0.6 nm (from local modes) and 10 nm (entanglement spacing). Experimental temperature dependencies of spectral widths are discussed with those obtained from the fluctuation approach. The report also contains a discussion of the presumed general scaling principle: At a given state, short mode length corresponds to high frequency (mobility) and vice versa.

Random ethyl acrylate-sulphonatopropylbetaine copolymers: 2. Dynamic mechanical properties

The dynamic mechanical properties of ethyl acrylate-sulphonatopropylbetaine copolymers (zwitterionomers) and their stoichiometric blends with lithium perchlorate were examined as a function of temperature and frequency from the glassy to the melt zones. By comparing with the behaviour of classical microphase-separated ionomers, it could be concluded that the zwitterionomers both with and without added salt display the characteristics of microphase separation, thus supporting previous small angle X-ray studies. The slope of the modulus curves in the glass transition region decreases with increasing zwitterion content (up to ≈ 10 mol%), the loss tangent intensity decreases and the rubbery plateau modulus increases. The addition of the salt affects the glass transition zone relatively little, but significantly extends the range of the rubbery zone, indicating that the electrostatic interactions in the zwitterionic phase are stable to higher temperatures or lower frequencies than in the absence of salt. Furthermore, a second dispersion region is clearly evident. Time-temperature superposition is applicable to all the copolymers, with and without salt, in the range of conditions studied.

Adhesion quality and extent of mesophase in particulates along the whole viscoelastic spectrum

AbstractThe extent of mesophase, which constitutes the boundary layer between the filler and the matrix of a composite, was defined in this paper along the whole viscoelastic spectrum of the polymeric substance constituting the matrix. The already established two‐term unfolding model1–3 was based on thermodynamic measurements of the heat capacity jumps at the glass transition regions of the filled and unfilled substances in order to define the extent of mesophase. In this paper we dispense ourselves with these delicate measurements, based on Lipatov's theory, and we use values taken from the storage and loss compliances of the composite and its constituents, along the whole viscoelastic spectrum of the composite. It was shown that the extent of the mesophase, necessary to sustain the shearing loading between phases, appearing at the glass transition zone, where the loss‐compliances presented their characteristic peaks of resonance, remained constant and almost independent of the respective values of mechanical properties at this critical region. It was shown that any set of values of the mechanical characteristic quantities, defining the composite and its constituents along the whole viscoelastic spectrum, is sufficient to evaluate the extent of the mesophase, and this boundary layer undergoes only negligible variations. Experimental evidence with typical E‐glass polystyrene and other particulates, at various volume fractions of the filler, yielded a satisfactory coincidence with the results derived by using Lipatov's theory and a constancy of the mesophase thickness along the whole temperature range.

Fine structure of the glass transition in amorphous polymers. Part 2. Interpretation of the G″ peak and J″ peak by different molecular motion

AbstractA survey of transition phenomenology in amorphous entangled polymers is presented. Using a general scaling hypothesis characteristic lengths are systematically attached to the different zones of the multiple glass transition. New experimental results show the location of caloric spectra between the peaks of G″ and J″ on the common frequency scale. A fine structure of the main transition is suggested. There are two components: (i) The proper glass transition zone with mode lengths up to 2 nm. This length scale is determined from the caloric spectra. (ii) The hindering zone of cooperative motion of polymer chain segments between entanglement points with mode lengths up to 10 nm.

Limits of the dynamic glass transition zone in an ethylene-vinylacetate copolymer

Previous measurements on an ethylene-vinylacetate copolymer are discussed anew in the framework of new concepts for the glass transition. The response signals (n.m.r., ultrasonics, dielectric polarization, torsion, and specific heat) are presented in a logarithm of frequency-temperature diagram. An arrangement of the response signals across the glass transition zone similar to that in other polymers was found. The absorption maximum temperature of ultrasonics and the temperature where the super-Lorentzian shape distinctively appears in the wl-n.m.r. spectra are located at the high frequency limitb of the glass transition zone corresponding to short-range correlated molecular motions.

Influence of the thermal history on dielectric properties of poly(vinyl chloride)

AbstractIn this paper, we report dielectric permittivity and loss of poly (vinyl chloride) samples that have received three different thermal treatments: (a) as received, (b) quenched from 110°C to 20°C and (c) slow cooled at 5°C/h. There are several observations: first, the secondary (β) loss peak‐is not representative of a simple mechanism of transition, in agreement with results of other authors (10), second, in the glass transition zone, there are clearly two peaks (α1 and α2)—α1, is a typical peak of an amorphous glass transition; the second, α2, has possibly a crystalline origin—and, third (and the most interesting fact), there is an increase of the loss tangent in the intermediate zone between α and β peaks showing a new relaxational peak with high activation energy (70 Kcal/mole), in agreement with dynamic mechanical results (6).