Polymer-plasticizer Systems Research Articles

Origin of Apparent Slow Solvent Dynamics in Concentrated Polymer Solutions

Due to their size disparity, the dynamics of solvent molecules in concentrated polymer solutions can be faster by several orders of magnitude compared to the polymer matrix. Despite these observations, past research suggests that a fraction of the solvent contributes to the slow dynamics on the timescale of the macromolecules. By combining depolarized dynamic light scattering and molecular dynamics simulations, we show that long-lived solvent–solvent cross-correlations cause these slow solvent contributions. They originate from the energy landscape that the solute imprints onto the neighboring solvent, leading to enhanced correlations between the positions and orientations of different solvent molecules until the solute relaxes. This mechanism explains the discrepancies found between the results of techniques probing collective and single-particle dynamics, such as dielectric and nuclear magnetic resonance spectroscopy. Our findings shed new light on how solvents behave in the vicinity of macromolecules and are of relevance for materials ranging from polymer-plasticizer systems to hydrated proteins.

Screening of polymer-plasticizer systems for propellant binder applications: an experimental and simulation approach

ABSTRACTComposite propellants of advanced propulsion systems demand high energy, sufficient structural integrity, and low temperature capability. Polymers and plasticizers contribute to all these three important characteristics of propellants and they need to be screened rigorously to achieve the most suitable combination. In this investigation, experimental and simulation studies were performed to predict the miscibility of polymers and plasticizers. Solubility parameter is the measure of miscibility of any two material of interest, evaluated by equilibrium swelling and intrinsic viscosity experiments. Further, molecular dynamic simulations were also used to evaluate the solubility parameter for a few important polymers and plasticizers that are used in rocket propellants. Flory-Huggins interaction parameter (χ) was calculated for identifying the suitable pair. Simulated and experimental data proved that energetic plasticizers are not miscible with hydroxyl terminated polybutadiene (HTPB), while n-Butyl-nitratoethylnitramine (Bu-NENA) was found to be the most suitable plasticizer for polymers containing nitrogen atoms. Therefore, this study finds its importance in identifying an appropriate plasticizer for any specific polymer, which is the foremost critical step for achieving successful propellant formulations with desired characteristics.

Non-polymeric asymmetric binary glass-formers. I. Main relaxations studied by dielectric, 2H NMR, and 31P NMR spectroscopy.

In Paper I of this series of two papers we study the main relaxations of a binary glass former made of the low-Tg component tripropyl phosphate (TPP, Tg = 134 K) and of a specially synthesized (deuterated) spirobichroman derivative (SBC, Tg = 356 K) as the non-polymeric high-Tg component for the full concentration range. A large Tg contrast of the neat components is put into effect. Dielectric spectroscopy and different techniques of 2H nuclear magnetic resonance (NMR) as well as of 31P NMR spectroscopy allow to selectively probe the dynamics of the components. For all concentrations, two well separated liquid-like processes are identified. The faster α2-process associated with the low-Tg component TPP shows pronounced dynamic heterogeneities reflected by quasi-logarithmic correlation functions at low TPP concentrations. The slower α1-process involves the reorientation of the high-Tg component SBC. Its correlation function is Kohlrausch-like as in neat glass formers. The corresponding time constants and consequently their glass transition temperatures Tg1 and Tg2 differ more the lower the TPP concentration is. Plasticizer and anti-plasticizer effect, respectively, is observed. At low temperatures a situation arises that the TPP molecules isotropically reorient in an arrested SBC matrix (Tg2 < T < Tg1). At T < Tg2 the liquid-like reorientation of TPP gets arrested too. We find indications that a fraction of the TPP molecule takes part in the slower α1-process of the high-Tg component. All the features known from polymer-plasticizer systems are rediscovered in this non-polymeric highly asymmetric binary mixture. In Paper II [B. Pötzschner et al., J. Chem. Phys. 146, 164504 (2017)] we study the secondary (β-) relaxations of the mixtures.

Dynamics of asymmetric non-polymeric binary glass formers-A nuclear magnetic resonance and dielectric spectroscopy study.

We study a dynamically asymmetric binary glass former with the low-Tg component m-tri-cresyl phosphate (m-TCP: Tg = 206 K) and a spirobichroman derivative as a non-polymeric high-Tg component (Tg = 382 K) by means of (1)H nuclear magnetic resonance (NMR), (31)P NMR, and dielectric spectroscopy which allow component-selectively probing the dynamics. The entire concentration range is covered, and two main relaxation processes with two Tg are identified, Tg 1 and Tg 2. The slower one is attributed to the high-Tg component (α1-process), and the faster one is related to the m-TCP molecules (α2-process). Yet, there are indications that a small fraction of m-TCP is associated also with the α1-process. While the α1-relaxation only weakly broadens upon adding m-TCP, the α2-relaxation becomes extremely stretched leading to quasi-logarithmic correlation functions at low m-TCP concentrations-as probed by (31)P NMR stimulated echo experiments. Frequency-temperature superposition does not apply for the α2-process and it reflects an isotropic, liquid-like motion which is observed even below Tg 1, i.e., in the matrix of the arrested high-Tg molecules. As proven by 2D (31)P NMR, the corresponding dynamic heterogeneities are of transient nature, i.e., exchange occurs within the distribution G(lnτα 2). At Tg 1 a crossover is found for the temperature dependence of (mean) τα 2(T) from non-Arrhenius above to Arrhenius below Tg 1 which is attributed to intrinsic confinement effects. This "fragile-to-strong" transition also leads to a re-decrease of Tg 2(cm - TCP) at low concentration cm - TCP, i.e., a maximum is observed in Tg 2(cm - TCP) while Tg 1(cm - TCP) displays the well-known plasticizer effect. Although only non-polymeric components are involved, we re-discover essentially all features previously reported for polymer-plasticizer systems.

Plasticization of cellulose and its derivatives

The relaxation transitions in binary blends of cotton cellulose nitrate with castor oil and of chitosan and wood cellulose with betulin were studied by low-temperature adiabatic calorimetry in the range 80–320 K and by differential thermal analysis in the range 80–620 K. The solubility of these “structural” plasticizers in the above polysaccharides was determined. The phase diagrams of the polymer-plasticizer systems were constructed and analyzed.

Discontinuous and heterogeneous glass transition behavior of carbohydrate polymer–plasticizer systems

In order to understand the glass transition properties of carbohydrate polymer–plasticizer systems, glass transition temperatures of dextrin–glucose and dextrin–maltose systems were investigated systematically using differential scanning calorimetry. The onset (Tgon) and offset (Tgoff) of the glass transition decreased with increasing plasticizer (glucose or maltose) content, and showed an abrupt depression at certain plasticizer content. The abrupt depression of Tgoff occurred at higher plasticizer content than that of Tgon. The glass transition was much broader for intermediate plasticizer content. From the enthalpy relaxation behavior of samples aged at various temperatures, it was found that two different glass transitions occurred contentiously in the broad glass transition. These results suggested that carbohydrate polymer–plasticizer systems can be classified into three regions: the entrapment of the plasticizer by the polymer, the formations of the polymer–plasticizer and plasticizer-rich domains, and the embedment of polymer into the plasticizer.

The application of miscibility parameter to the measurement of polymer-plasticizer compatibility

Poly(vinyl butyral) (PVB)was found to precipitate from a clear plasticizer solution as it was cooled and this unique temperature (TP) (UCST) was noted. Poly(vinyl chloride) (PVC) did not precipitate and in this case the temperature at which it went into solution (TS) was noted. A quantity, referred to previously as the miscibility parameter (MP), is an excellent relative quantitative indicator of the miscibility of poly(vinyl butyral) and poly(vinyl chloride) with plasticizers as demonstrated from a correlation of MP with TP and Ts. Each polymer-plasticizer system is unique. A plot of MP vs. TP or TS for several polymer-plasticizer systems allows additional plasticizers to be evaluated for their plasticizing ability from calculated MP's without resorting to additional experimental measurements. The absolute level of MP acceptability is dictated by the application. For PVB, a copolymer which precipitates from a plasticizer/solvent, the interaction parameter chi(χ) is chi critical (\(\chi _{cr} \)) at the TP of PVB in plasticizer. Since TP approximates the Θ temperature in the limit of molecular weight, the temperature of precipitation or Θ temperature, the solvent volume VS, and the \(\chi _{cr} \), define the theoretical MPcr as given by \(\frac{{(\chi _{cr} - 0.35)RT}}{{V_S }}\). All MPcr s were≈0.3 which represents the critical MP above which phase separation occurs. The low molecular weight plasticizers were estimated to drastically increase MPcr to about 1 for acceptable application compatibility and this value compares favorably with (PVB) acceptability for a wide range of plasticizer-PVB combinations based on a plasticizer DP of about 20.

Prediction of compatibility in polymer-plasticizer systems

The UNIFAC group contribution method has been used to calculate the free energy of mixing versus composition curves for several polymer-plasticizer systems, including phthalates and stearates with poly(vinyl chloride). The solubility of the plasticizer has been calculated from the curves and compared with experimental results. Although the absolute values of the solubilities were underestimated, the relative solubilities amongst groups of compounds were predicted well, so that the method may be useful in the prediction of compatibility in these systems.

DSC study of the effects of plasticization of polymers and the possibilities of their prediction

DSC has been used to investigate a number of polymer-plasticizer systems with change in the concentration of the latter from 2 to 50% by weight. From the notion of the common nature of α and β relaxations in flexible chain polymers the authors propose an interpretation of the effects observed and the relations for predicting the effects of plasticization starting from the size of the statistical segment (or the temperature of the β transition) of the polymer and the solubility parameters of polymer plasticizer.

Reversed gas chromatography of polymeric systems. Review

Problems of correctly USAing the method of reversed gas chromatography associated both with the features of conducting the experiment and interpretation of the results are considered with examination of the role of the nature of the sorbate. The USAe of this method, for investigating the thermodynamics of the interaction in polymeric systems is dUSAcUSAsed, in particular, for determining the thermodynamic compatibility in polymer-polymer and polymer-plasticizer systems, the parameters of the solvent-polymer interaction and the parameters of the solubility of polymers and plasticizers. The possibilities of the method are analysed and its USAe described in study of phase and relaxational transitions in polymeric systems, determination of the glass transition and melting points of polymers and their degree of crystallinity. The prospects of the method both in the direction of extending the areas of its relations for determining the physicochemical constants of polymers from the chromatographic data are considered.

Study of phase equilibrium by refractometry and the determination of second virial coefficients of polymer-plasticizer systems

It was found possible to use refractometry for plotting phase diagrams of polymer-plasticizer systems when refractive indices of components are close and the cloud point method cannot be used. Second virial coefficients for the plasticized system were determined for the first time by light scattering.

Evaluation of compatibility and phase equilibrium in polymer-plasticizer systems

Comparative results are given of the phase condition in systems of polyvinyl-chloride-polyester plasticizers studied by optical interferometry, NMR, thermo-optical and thermomechanical methods and cloud points. It was shown that solubility values of components obtained by various methods coincided.

The phase diagram for polymer-crystalline plasticizer systems

A method of plotting the phase diagram of polymer-crystalline plasticizer systems, based on the thermodynamic properties of polymer concentrates in the temperature range above the melting point of the plasticizer, is suggested. The results of calculating the phase equilibrium temperature by the suggested method are compared with those of the direct melting temperature ( T m) determinations by the optical method. Our method has been found suitable for a rapid and fairly accurate determination of the crystallization temperature ( T cryst) of the plasticizer from its polymer solution. The use of our method in the phase equilibrium study does away with the need for a lengthy thermostating of the solution at a low temperature; it is suitable for the investigation of a wide range of polymer-plasticizer systems.

Polymer miscibility in organic solvents and in plasticizers—a two‐dimensional approach

AbstractFrom thermodynamic considerations based on associated solution models and from the Hansen's three‐dimensional solubility parameter concept, it is found that the solvent power of an organic liquid for a given polymer can be characterized by two parameters, δh and χH, where δh is the hydrogen‐bonding solubility parameter of the liquid, and χH is a term which takes account of the dispersion and polar interactions between the liquid and the polymer and of the effects due to temperature and molecular size of the liquid. It is also found that Hansen's solubility sphere for the polymer can be represented as a solubility circle in the proposed χH−δh plane. The proposed approach is applied successfully to polymer–plasticizer systems.

Glass Transitions in Polymer-Plasticizer Systems

Abstract A summary of the investigation with the three types of diluent can he made with reference to Table VI. Hydrocarbon oils are more effective low-temperature plasticizers the lower their temperature coefficient of viscosity. Dibutyl sebacate is representative of crystallizing diluents. Such compounds are ineffective low temperature plasticizers unless, (a) at equilibrium, favorable thermodynamic parameters permit their existence as liquids in the polymer, or (b) the plasticizers tend to exist as the nonequilibrium, supercooled, liquids in the transition temperature region of the polymer. Particularly desirable are liquids with low apparent activation energies of viscosity and high miscibility with polymer. The four ester plasticizers, tributoxyethyl phosphate, dibutyl phthalate, dioetyl adipate, and dioctyl sebacate, have temperature coefficients of viscosity corresponding to a virtually common activation energy of 4.5 kilocalories. Here, the magnitude of diluent-polymer interaction, as indicated by volume increase measurements, dictates the order of effectiveness of the ester.

Glass Transitions in Polymer-Plasticizer Systems

Glass temperatures (Tg) of pure and plasticized elastomers have been obtained from dilatometric measurements and from the temperature dependence of angular response to torsional stress. The depression of Tg by a diluent, determined by either method, is a convenient measure of its plasticizing ability. Some compounds which are normally crystalline above Tg of the pure polymer, but nevertheless compatible with the polymer, depressed the glass temperature. The polymer-diluent compatibility at low temperatures results either from supercooling, the depression of the melting point of diluent by polymer, or possibly a combination of both phenomena. Tg decreased as the diluent loading increased to a more or less well defined limit, beyond which further dilution had negligible effect. For diester type plasticizers both ΔTgmax and the limiting diluent concentration were linear functions of compatibility, as defined by the extent to which standard polymer vulcanizates were swollen by plasticizer. The viscosities and apparent activation energies for flow (Ev) of hydrocarbon oils correlated with their low temperature plasticizing efficiencies. Pour points and glass temperatures may also be significant properties of the diluents. It is concluded that low Ev and high solvency for polymer are desirable properties for low temperature plasticizers.

Effect of Polar Vapours on the Direct-Current Conductance of Keratin and Nylon

Fuoss1,2 and others, have shown that in polar polymer-plasticizer systems there exists some correlation between the frequency of the maximum power-loss absorption, and (a) the elastic moduli, and (b) the conductance. That is:

Characterization of cellulose derivatives by solution properties: Plasticizers as solvents

AbstractThe dimensional stability of molded thermoplastic articles is a function of the extent to which stresses are frozen into them on molding. The kind and amount of plasticizer included in the material to be molded may be varied to minimize the amount of these stresses. Methods of obtaining fundamental information about polymer–plasticizer systems from simple measurements of viscosity and osmotic pressure are suggested in this paper, and some results for cellulose derivatives are given.