Blends of Amorphous Polymer Components

There are only a few compatible polymer blends without a pronounced morphology, such as PS/PPO or SAN/PMMA. The morphology of the other blends of amorphous polymers is determined by their incompatibility and processing conditions. Occasionally it can be difficult to reveal the morphology of an amorphous polymer blend in detail if there are no clear interface boundaries or the components show no different staining abilities in such cases some special preparation techniques are used. 

The following polymer blends are presented PC/MBS, PC/ABS, PC/PMMA, PMMA/PB, PC/SAN, and epoxy/SBS 

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After having studied in our laboratory, polymer blends of amorphous polymers poly-c-caprolactone and poly (vinyl chloride) (1,2) (PCL/ PVC), blends with a crystalline component PCL/PVC (3,4), poly(2,6-dimethyl phenylene oxide) (PPO) with isotactic polystyrene (i-PS) (5) and atactic polystyrene (a-PS) with i-PS (6), we have now become involved in the study of a blend in which both polymers crystallize. The system chosen is the poly(1,4-butylene terephthalate)/poly(ethylene terephthalate) (PBT/PET) blend. The crystallization behavior of PBT has been studied extensively in our laboratory (7,8) this polymer has a... 

Two-component one-phase systems (miscible blends of amorphous polymers)... 

Blends of amorphous polymers with crystaUine polymers (Fig. 6.3) also show differences between miscible and immiscible components. In this comparison, the crystaUine polymer has a lower Tg than the amorphous polymer and exhibits a crystalline modulus plateau between the Tg and the Tm- The phase separated blend shows both TgS, with a modulus plateau between... 

Since interactions at the molecular level between polymer components in the blends occur only in the amorphous phase, it is reasonable to assume that these effects are due to kinetic factors and, in particular, to the influence of a polymer component on the nucleation or crystallization kinetics of the other one. 

The phase behavior of polymer blends comprising amorphous polymers is experimentally well accessible in a window which is bounded at high temperatures by the thermal decomposition temperature of the polymer components and at low temperatures by the glass transition temperature of the system (cf. Fig. 1). Below the glass transition temperature the phase behavior can be estimated only tentatively. 

Compatible Polyblends. When the polymeric materials are compatible in all ratios, and/or all are soluble in each other, they are generally termed polyalloys. Very few pairs of polymers are completely compatible. The best known example is the polyblend of polyCphenylene oxide) (poly-2,6-dimethyl-l,4-phenylene oxide) with high-impact polystyrene (41). which is sold under the trade name of Noryl. It is believed that the two polymers have essentially identical solubility parameters. Other examples include blends of amorphous polycaprolactone with poly(vinyl chloride) (PVC) and butadiene/acrylonitrile rubber with PVC the compatibility is a result of the "acid-base" interaction between the polar substituents (1 ). These compatible blends exhibit physical properties that are intermediate to those of the components. 

To be able to measure the osmotic pressure n, a semipermeable membrane that permits passage of the solvent molecules but not the solute molecules is needed. This can, in practice, be realized only when there is a large disparity between the sizes of the solute and solvent molecules, as in a solution of a polymer in a small-molecule solvent. However, the existence of osmotic pressure can be envisioned, at least mentally, with any kind of solution, such as a solution of two small-molecule liquids or a miscible blend of two polymers. Equation (6.6) is thus valid for any two-component (amorphous) system, as long as it is in equilibrium and classical thermodynamics is applicable to it. For applications to these general cases, it is more convenient if Equation (6.6) is reformulated in terms of the free energy of mixing and no explicit reference to osmotic pressure is made in it. 

Common examples of miscible blends are ethylene-propylene copolymers of different composition that result in an elastomer comprising a semicrystalline, higher ethylene content and an amorphous, lower ethylene content components. These blends combine the higher tensile strength of the semicrystaUine polymers and the favorable low temperature properties of amorphous polymers. Chemical differences in miscible blends of ethylene-propylene and styrene-butadiene copolymers can also arise from differences in the distribution and the type of vulcanization site on the elastomer. The uneven distribution of diene, which is the site for vulcanization in blends of ethylene-propylene-diene elastomers, can lead to the formation of two distinct, intermingled vulcanization networks. 

Blend properties depend strongly oti which polymer is the continuous phase. The majority of commercially important compatibilized blends of semiciystalline polymers with amorphous polymers are prepared in compositions such that the semicrystalline component is the matrix and the amorphous component is the dispersed phase. Such blends show adequate solvent resistance since in this morphology the surface consists largely of the dominant, matrix phase of semiciystalline polymer. 

In immiscible blends, the t-T principle does not hold. For immiscible amorphous blends, it was postulated that two processes must be taken into account the t-T superposition and the aging time (Maurer et al. 1985). On the other hand, in immiscible blends, at the test temperature, the polymeric components are at different distances from their respective glass transition temperatures, T — Tgi T — Tg2. In blends of semicrystalline polymers, such as PE/PP, the superposition is limited to the molten state, within a narrow, high temperature range (Dumoulin 1988). 

The morphology of polymer blends that consist of one crystallizable component and one noncrystallizable amorphous compound has been studied in detail by several groups, with attention focused on various crystalline-amorphous combinations and compositions [48-50]. A single l -value always characterizes crystalline-amorphous blends, and is typically an intermediate value between that of the pure homopolymers the exact Tg depends on the weight fraction of the two polymers, however. Flighly crystalline polymers that have been studied in crystalline-amorphous blends include poly(e-caprolactone) (PCL), poly(phenylene oxide) (PPO), poly(ethylene oxide) (PEO), poly(butyleneterephthalate) (PBT), poly(ethyleneter-ephthalate) (PET), and high- and low-density polyethylene (HOPE and LDPE, respectively), whereas poly(vinyl chloride) (PVC) and poly(methyl methacrylate) (PMMA) are examples of amorphous polymers used for blending studies (51j. 

Blends comprised of amorphous, low Tg polymers are of primary interest for elastomeric type applications, of which the large tire market commands considerable interest. This section will consider blends of elastomeric polymers, generally low Tg, amorphous blends. In specific cases, low modulus, crystalline polymer blends (such as ethylene copolymers) with other elastomeric materials will be included. Also blends containing crystalline polymer, where the primary component of the blend is the elastomeric component and the blend is considered an elastomeric material, will be discussed. Specifically, dynamic vulcanized blends such as polypropylene/ethylene-propylene rubber blends will be included in this section. 

For a binary miscible blend (A amorphous polymer, B crystalline polymer) the glass transition is intermediate between those of plain components (Tg, Tje) and thus the crystallization range—and the crystallization behavior— will depend on the glass transition of the amorphous component (Tja) (Fig. 10.2) [19]. In fact, if FgA is lower than T,b, the glass transition of the crystallizable polymer, the crystallization window of the blend (T°-Tg), where T° represents the equilibrium melting point of B in the blend, is larger than that of the neat crystallizable component (T b -TgB), and the ability to crystallize is enhanced. On the contrary, if TgA... 

Figure 10.2 Crystallization temperature ranges for crystalline/amorphous miscible polymer blends (A amorphous polymer, B crystalline polymer) as a function of volmne fraction of the crystalline component. Tg, glass transition temperature of blend TgA < (sohd Une), (dashed line). T, ...

Crystallizable immiscible blends may consist of both crystallizable polymer components (crystalUne/crystaUine blends), or of one crystalline component (crystalline/ amorphous blends), which can be present as matrix phase or as dispersed phase. The crystallization behavior and the structure-properties relationships of immiscible blends of various polymer classes have been investigated by numerous authors a comprehensive overview of the crystallization phenomena and morphological characteristics of these systems has been reported in References [19,72],... 

Chapter 3 provides a brief review of recent developments in areas of amorphous polymer blends. Differential mixing, chain dynamics, and glass transition properties for individual polymer components in miscible binary blends, as well as new methods to experimentally acquire such information, are considered. Miscible blend dynamics and length scales of mixing of amorphous polymer blends are discussed. Amorphous biopolymer blends involving polymers obtained from renewable feedstocks is also briefly reviewed. 

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