Relaxation Of Copolymers Research Articles

Dynamics of polystyrene-block-poly(methylmethacrylate) (PS-b-PMMA) diblock copolymers and PS/PMMA blends: A dielectric study

The dielectric relaxation properties of poly(methylmethacrylate) (PMMA), polystyrene-block-poly(methylmethacrylate) (PS-b-PMMA) diblock copolymers and PS/PMMA blends were investigated by broadband dielectric spectroscopy in the frequency range from 0.01Hz to 1MHz and temperature range from −40 to 150°C. By parametric fitting techniques, an α relaxation process associated with the glass transition, the secondary β relaxation process due to localized motions of side groups, and the conductivity at low frequencies were identified and modeled for all samples. Based on the modeling results, the characteristic relaxation time, fragility index, activation energies, and relaxation strength of PMMA and PS-b-PMMA diblock copolymers were compared. It was found that the structural α relaxation of the copolymers was suppressed compared with that of PMMA. The covalent chemical bonds that link PS and PMMA blocks in the diblock copolymers are therefore assumed to dominate over the PMMA–PMMA, PMMA–PS and PS–PS bonds that exist in the PMMA and PS/PMMA samples, accounting for the suppression of the cooperative structural α relaxation and for larger activation energies of the localized β relaxation of the diblock copolymers. Further, the activation energy barriers of localized β relaxations in the copolymers are observed to increase, above the glass transition temperature.

Compositional variation of the glass transition and the associated dielectric relaxation in copolymers of vinylidene fluoride and trifluoroethylene

The glass transition temperature ( T g) of a series of statistical copolymers of vinylidene fluoride and trifluoroethylene has been determined experimentally. Its variation as a function of the chemical composition of the copolymers has been discussed based on the Fox, Di Marzio and Couchman equations. Using T g = −42°C for poly(vinylidene fluoride) and a value of T g usually reported for polytrifluoroethylene ( T g = 31°C, a significant discrepancy has been obtained between experimental and predicted values. Thus, it has been suggested that the glass transition temperature for polytrifluoroethylene is located in the range of −20 to −10°C, which is consistent with Wunderlich's thermal analysis data.

Dielectric relaxations in copolymers of vinylidene fluoride

Abstract Ferroelectric copolymers of vinylidene fluoride with trifluoroethylene and tetrafluoroethylene have been investigated by means of broad band dielectric spectroscopy (10 mHz-1 GHz) at temperatures covering the ferroelectric, paraelectric and molten phases as well as the glassy and rubbery states. The low-temperature relaxation spectra near Tg (∼ -40°C) consist of two processes; a WLF-type segmental mode and an Arrhenius type local mode. These processes are unified near room temperature to be transferred continuously to the high-temperature process. After showing a peak at the Curie point Tc (70–140°C), the relaxation strength decreases gradually in the paraelectric phase until the melting point Tm (∼ 150°C) is reached, where it decreases rapidly. It is found that the relaxation time in the molten phase lies on the Arrhenius line extended from the low-temperature local mode process. This implies that related molecular motions arise from a common elementary process. Departure from this Arrhenius lin...

Aliphatic polyesters as models for relaxation processes in crystalline polymers: 2. Dielectric relaxation in copolymers of adipic acid with 1,6 and 2,5-hexanediols

Linear aliphatic polyesters are interesting model systems for studying relaxation processes in crystalline polymers because they show both γ and β relaxations in a prominent way whereas in many highly crystalline linear polymers (LPE, POM etc.) the β process is suppressed and, unlike branched polyethylene which also has a prominent β process, the polyesters have no chemical structural complications due to branches. To study the hypothesis that variations in the appearance of the β (glass-rubber) relaxation are due to varying degrees of immobilization of long-range segmental motion in the amorphous fraction by the presence of the crystal phase measurements are made of dielectric relaxation in a series of the title copolymers that span the crystallinity range of zero (homopolymer with 2,5-hexanediol) to ≈60% (homopolymer with 1,6-hexanediol). The data are analysed in terms of the effect of degree of crystallinity on both kinetic parameters (activation energies and relaxation width) and equilibrium parameters (dipole correlation factor deduced from relaxation strength). In the latter case special care is taken to properly assess composite mixture effects on the relaxation strength. The shape of the β process is extremely sensitive to the degree of crystallinity, broadening greatly with the onset of the presence of crystallinity and increasing in broadness with increasing degree of crystallinity. Although less sensitive than the width, the β relaxation dipole correlation factor depends on crystallinity also, the availability of chain configurations within the amorphous fraction being noticeably reduced in the crystalline specimens. The γ process kinetic parameters are relatively insensitive to the presence of the crystal phase, a behaviour consistent with a localized motion mechanism. The γ relaxation strength correlates with the process having an amorphous phase origin if it is assumed that the 2,5 diol units contribute less to relaxation strength than the 1,6 diol units; a result also consistent with the localized motion concept.

Aliphatic polyesters as models for relaxation processes in crystalline polymers: 3. Mechanical relaxation in copolymers of adipic acid with 1,6 and 2,5-hexanediols

The shear moduli of a series of the title polyesters spanning a crystallinity range of 0–60% have been measured as a function of temperature at ≈1 Hz using a torsion pendulum. The experimental isochronal temperature scans of G′ and G″ are fitted to phenomenological equations. With only minor adjustments, the same relaxation spectrum parameters as found dielectrically for these polymers (relaxation shape, central relaxation times) fit the mechanical data. Thus, for the β (glass—rubber) relaxation in the amorphous fraction the broadness is very sensitive to the presence of the crystal fraction and becomes increasingly broad as the degree of crystallinity increases. In contrast, the γ process dynamic behaviour is insensitive to the presence of and degree of crystallinity. Unrelaxed and relaxed moduli values are determined for the γ and β processes. A composite model approach is used to determine bounds on the amorphous-phase unrelaxed and relaxed γ and β moduli from the bulk specimen values. As was the case dielectrically, the γ process, in addition to being assigned to the amorphous fraction, has a strength that depends on the diol composition also. The relaxed ( γ + β) amorphous-phase rubbery shear modulus is bound reasonably well from application of the composite model to the bulk specimen values and is assigned the value 100 ± 20 MPa at 250 K. It decreases relatively strongly with increasing temperature.

Dielectric relaxation in copolymers of vinylidene fluoride and hexafluoropropylene

Dielectric relaxation in copolymers of vinylidene fluoride and hexafluoropropylene

Dielectric relaxation of copolymers of methylmethacrylate with α-methylstyrene

Dielectric relaxation of copolymers of methylmethacrylate with α-methylstyrene

Dielectric relaxation in copolymers of methyl methacrylate with methyl acrylate

Dielectric relaxation in copolymers of methyl methacrylate with methyl acrylate

Relaxations in Copolymers of Tetrafluoroethylene and Hexafluoropropylene

The modulus and internal friction of copolymers of tetrafluoroethylene and hexafluoropropylene (HFP) were measured with longitudinal waves at a frequency of 12 Mc between −80° and 260°C. The dielectric dissipation factor was measured at 0.1, 1.0, 10, and 100 kc between −180° and 200°C. Samples included copolymers with compositions ranging from pure polytetrafluoroethylene to pure polyhexafluoropropylene. A relaxation at −10°C (12 Mc) in polytetrafluoroethylene shifts to higher temperatures with increasing HFP content. Data suggest that 5 to 13 main-chain carbon atoms are involved, a size consistent with assigning the relaxation to motions of a small point defect in the crystal lattice. Molecular models indicate that the effects of these defects are reduced by perfluoromethyl groups which enter the lattice as another kind of point defect. Dielectric and mechanical relaxation temperatures agree approximately and yield the same activation parameters. These parameters exhibit no significant variation with HFP content and follow a relation which applies to activated processes in inorganic solids. The relation also obtains for a relaxation at 197°C (12 Mc) in polytetrafluoroethylene. This relaxation shifts to lower temperatures and the activation enthalpy decreases with increasing HFP content. Dielectric data do not exhibit the relaxation, and heating polytetrafluoroethylene near the melting point does not introduce sufficient polar groups to alter this result. Another relaxation observed below −150°C in the dielectric but not in the ultrasonic data is independent of polymer density and HFP content.