BACKGROUND OF THE INVENTION Field of the Invention [0001] This invention relates generally to a textile-reinforced, elastorneric poljurethane articles for dynamic applications, more paiticularly to a power transmission r· . ! I belt with a plasticized polyurethane/urea elastomer and carbon fiber tensile cord, and specifically to an improved polyurethane/urea elastomer composition which improves flex fatigue resistance of carbon fiber tensile cord in polyurethane belts. Description of the Prior Art [0002] It is generally known that certain additives, known as plasticizers or diluents, when added to polyurethane prepolymers will reduce the viscosity of these prepolymers. Unfortunately, a simultaneous effect of these additives is to significantly reduce certain cured properties ofthe polyurethane elastomer such as hardness, tensile modulus, and tear strength. [0003] U.S. Pat. No. 5,907,014 discloses polyurethane prepolymers with improved wettability without sacrificing physical properties. Prepolymers with dibasic esters, preferably aliphatic dialkyl esters where the alkyl group is C1 to C7, and diamine or polyol curatives are applied to one or more fabric plies and are subsequently cured at the appropriate temperature and time to create a finished product, i.e., a conveyor belt. [0004] U.S. Pat. No. 6,964,626, ti1e contents of which are hereby incorporated herein in its entirety, discloses improved polyurethane/urea elastomers having high temperature stability to about 140-150°C and low temperature flexibility at about -35-(- 40)0C, for use in dynamic applications. These elastomers are useful for application in belts, specifically in automotive timing or synchronous belts, V-belts, multi-V-ribbed or micro-ribbed belts, flat belting and the like. The polyurethane/urea elastomers are prepared by reacting polyisocyanate prepolymers with symmetric primary diamine chain extenders, mixtures of symmetric primary diamine chain extenders and secondary diamine chain extenders, or mixtures of symmetric primary diamine chain extenders and 2 non-oxidative polyols, which are all chosen to eliminate the need for catalysts via standard molding processes, and to improve phase separation. The polyisocyanate prepolymers are reaction product~ of polyols which are non oxidative at high temperatures,. such as polycarbonate polyols, polyester polyols, or mixtures thereof, with organic polyisocyanates which are either compact, symmetric and aromatic, such as paraphenylene diisocyanate, 1,5-naphthalene diisocyanate, and 2,6-toluene diisocyanate, or are aliphatic and possess trans or trans,trans geometric structure, such as trans-1,4- cyclohexane diisocyanate and trans,trans-4,4'-dicyclohexylmethyl diisocyanate. SUMMARY [0005] The present invention is directed to polyurethane/urea elastomers, power transmission belts, and methods which provide improved room temperature and hightemperature flex fatigue resistance and improved dynamic load performance. [0006] The present invention is directed to a power transmission belt having a main belt body portion of elastomeric material, a tensile reinforcement disposed in said body portion, and a sheave contact portion integral with said main body portion. The elastomeric material includes the reaction product of a polyisocyanate prepolymer composition and a chain extender. The polyisocyanate prepolymer composition is prepared by reacting (i) a diisocyanate selected from the group consisting of (a) paraphenylene diisocyanate, (b) 2,6-toluene diisocyanate, and (c) a cycloaliphatic diisocyanates with trans or trans, trans geometric structure; (ii) a polyol substantially free of moieties oxidative at less than about l50°C and selected from the group consisting of (a) polycarbonate polyols, (b) polyester polyols, and (c) mixtures of said polycarbonate polyols and said polyester polyols; and (iii) at least one trio! crosslinker selected from the above group ofpolyols. Either before or after reacting the polyisocyanate prepolymer composition, added there to is a plasticizer selected from the group consisting of alkylether di-alkylesters, polyalkyl-ether di-alkylesters, and polyethylene glycol di-alkylesters. The chain extender is selected from the group consisting of aromatic symmetric primary diamine chain extenders, mixtures of said aromatic symmetric primary diamine chain extenders and aromatic secondary diamine chain extenders, and mixtures of one or more of said aromatic symmetric primary diamine chain extenders and one or more polyols selected from those used to make the prepolymer. 3 i [0007] In various embodiments, the tensile reinforcement may be a carbon tiber tensile cord. Reinforcem.ent may include a textile fabric, which may cover a surface of the belt, and maybe a laminate of fabric with a polymer film. [0008] In various embodiments, the trio! content in the prepolymer may be quite high, from 2% to 100%, preferably from 5% to 35%, or from about 15% to 25% by weight based on the total polyol content. The plasticizer content in the prepolymer may be from about 5% to about 30% or from 5% to about 15% by weight based on the total .. prepolymer ~eight. The plasticizer may be polyethylene glycol di-2-ethylhexoate and may have q-molecular weig~t in the range from 300 to about 700 .. ' [0009] In various embodiments, the polyol may be a mixture of polycaprolactone diol of molecular weight from about 500 to about 4000, or from 1500 to 2500, and polycaprolactone trio! of molecular weight from about 500 or I 000 to about 4000 or from 2500 to 3500. [0010] The invention is also directed to a method of preparing the aforementioned polyurethane elastomer and molding an article molding an article subject to dynamic loading in use selected from the group consisting of a belt, belting, a flexible coupling sleeve, and a hose and embedding in or on a surface of said article a textile reinforcement having interstices between fibers thereof, so that the urethane penetrates at least some of the interstices before or during the molding. [0011] The advantages of embodiments of the invention include improved pot life, mold filling rate, and other processing characteristics of the polyurethane, improved tensile cord strength retention in belt flex testing, improved hot load and flex performance, and improved belt durability. [0012] The foregoing has outlined rather broadly the features and technical advantages of the present invention in order that the detailed description of the invention that follows may be better understood. Additional features and advantages oft he invention will be described hereinafter which form the subject of the claims of the invention. It should be appreciated by those skilled in the art that the concepti oil and specific embodiment disclosed may be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes oft he present invention. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope ofthe invention as set forth in the appended claims. The novel features which are believed to be characteristic of the invention, both as to its organization and method of operation, together with further objects and advantages will be better understood from the following description when considered in connection with the accompanying figures. It is to be expressly understood, however, that each of the figures is provided for the purpose of illustration and description only and is not intended as a definition of the limits of the present invention. BRIEF DESCRIPTION OF THE DRAWINGS [0013] ,..-The accompanying drawings, which are incorporated in and form pari of the specification in which like numerals designate like parts, illustrate embodiments of the present invention and together with the description, serve to explain the principles of the invention. In the drawings: [0014] FIG. 1 is a fragmented perspective view, with parts in section, of a timing belt constructed in accordance with an embodiment of the present invention; [0015] FIG. 2 is a fragmented perspective view, with parts in section, of a V-belt constrncted in accordance with an embodiment ofthe present invention; [0016] FIG. 3 is a fragmented perspective view, with parts in section, of a multi-Vribbed belt constructed in accordance with an embodiment of the present invention; [0017] FIG. 4 is a graph of hot stress-strain data of polyurethane/urea elastomer compositions according to embodiments of the invention and comparative compositions; [0018] FIG. 5 is a schematic of a RTFF test rig used for evaluating a characteristic of a belt embodiment of the invention; and [0019] FIG. 6 is a schematic of a HTFF test rig used for evaluating a characteristic of a belt embodiment of the invention. DETAILED DESCRIPTION [0020] The present invention is directed to a power transmission belt having a main belt body portion of elastomeric material, a tensile reinforcement disposed in said body portion, and a sheave contact portion integral with said main body portion. According to the invention, the elastomeric material includes the reaction product of a plasticized, polyisocyanate prepolymer composition and a chain extender. The polyisocyanate 5 prepolymer composition is prepared by reacting (i) a diisocyanate selected from the group consisting of(a) para-phenylene diisocyanate, (b) 2,6-toluene diisocyanate, and (c) a cycloaliphatic diisocyanates with trans or trans, trans geometric structure; (ii) a polyol substantially free of moieties oxidative at less than about 150°C and selected from the group consisting of(a) polycarbonate polyols, (b) polyester polyols, and (c) mixtures of said polycarbonate polyols and said polyester polyols; and (iii) at least one trio! crosslinker selected from the above group ofpolyols. Either before or after reacting the polyisocyanate prepolymer composition, added there to is a plasticizer selected from the group consri·s·t·i·n g of allcyi-ether di-allcylesters; polyalk)'i-ether di-allcyfestel"S, and ~ , polyethylene glycol di-alkylesters. The chain extender is selected from the group consisting of aromatic symmetric primary diamine chain extenders, mixtures of said aromatic symmetric primary diamine chain extenders and aromatic secondary diamine chain extenders, and mixtures of one or more of said aromatic symmetric primary diamine chain extenders and one or more polyols selected from those used to make the prepolymer. [0021] Referring to FIG. I, a typical timing belt 10 is illustrated. Belt I 0 includes elastomeric main body portion 12 and sheave contact portion 14 positioned along the inner periphery of main body portion 12. This particular sheave contact portion 14 is in the form of alternating transverse teeth 16 and land portions 1& which are designed to mesh with a transverse-grooved pulley or sprocket. Tensile layer 20 is positioned within main body portion 12 for providing support and strength to belt 10. In the illustrated form, tensile layer 20 is in the form of a plurality of cords 22 aligned longitudinally along the length of main body portion 12. It should be understood that, in general, any,type of tensile layer 20 known to the art may be utilized. Moreover, any desired material may be used as the tensile member, such as cotton, rayon, nylon, polyester, aramid, steel, carbon, PBO, and even discontinuous fibers oriented for low load carrying capability. In the preferred embodiment of FIG. 1, tensile layer 20 is in the form of illustrated tensile cords 22 made from carbon fiber, which preferably may be a twisted yarn of continuous carbon filaments. [0022] Reinforcing fabric 24 may be utilized and intimately fits along the alternating teeth 16 and alternating land portions 18 of belt 10 to form a face cover or tooth cover for the sheave contact portion. This fabric may be of any desired configuration such as a conventional weave consisting of warp and weft threads at any 6 ~-=--~-- desired angle or may consist of warp threads held together by space pick cords, or of a knitted or braided configuration, or a nonwoven, and the like. More than one ply of fabric may be employed, or combinations of different fabric types. If desired, fabric 24 may be cut on a bias so that the strands form an angle with the direction of travel of the belt. Conventional fabrics may be employed using such materials as cotton, polyester, polyamide, acrylic, aramid, polyketone, polyarylene sulfide, hemp, jute, fiberglass, and various other natural and synthetic fibers including blends or combinations thereof. In a preferred embodiment of the invention, fabric layer 24 consists of an expansible wearresistant fabr-ric· in which at least one of the warp or weft threads is m·ade of nylon.· In the ~ ~ ~ . preferred form, fabric layer 24 is made from a nylon 66 stretch fubric, and presents an elastomer-free (polyurethane/urea-free) surface for engaging cooperating drive sheaves. The elastomer-free surface may include a polymeric film laminated to the fabric. [0023] Referring to FIG. 2, standard notched V-belt 26 is illustrated therein. V -belt 26 includes an elastomeric body portion 12 similar to that ofF! G. 1 and tensile reinforcement member 20 in the form of cords 22, also similar to that as illustr·atcd in FIG. I. The elastomeric body 12 and cords 22 of V -belt 26 are constructed from the same materials as described above for FIG. 1. [0024) V-belt 26 also includes sheave contact portion 14 as in the power transmission belt of FIG. 1. In this embodiment, however, sheave contact portions 14 are the two juxtaposed sides of the belt, designed to wedge into a V-sheave. The bottom surface ofV-belt 26 is in the fonn of alternating notch depression surfaces or troughs 28 and projections 30. These alternating notched depression surfaces 28 and projections 30 preferably follow a generally sinusoidal path as illustrated which serves to distribute and minimize bending stresses as ihe sheave contact portion 14 passes around pulleys and sheaves. However, troughs 28 and projections 30 are optional. [0025] Referring to FIG. 3, multi-V-ribbed belt 32 is illustrated. Multi-V-ribbed belt 32 includes main elastomeric body portion 12 as in the belts of FIG's 1 and 2 and also includes tensile reinforcement member 20 preferably in the fonn of cords 22, also as previously described. Longitudinally grooved sheave contact portion 14 is in the fonn of a plurality of raised areas or apexes 36 alternating with a plurality of trough areas 38 having oppositely facing sides which define driving surfaces 34 of the belt 32. In each of these instances of FIG's 1-3, sheave contact portion 14 is integral with main body portion 12 and formed from the same elastomeric material to be described in greater detail below. 7 While the present invention is illustrated with reference to the embodiments shown in FIG's 1-3, it should be understood that the present invention is not to be limited to these particular embodiments or forms as illustrated but rather is applicable to any belt construction within the scope of the claims as defined below. [0026] The polyurethane/urea elastomers of the present invention are reaction products of plasticized polyisocyanate prepolymers with diamine chain extenders, via standard molding processes. The polyisocyanate prepolymers are reaction products of mixtures ofpolyols, including at least one trio!, nonoxidative up to 150°C, such as polycarbon.ate·polyols or polyester polyols, with either symmetric; compact, aromatic diisocyanates, such as PPDI, or trans- or trans,trans-geometrically configured aliphatic diisocyanates, such as trans-1,4-cyclohexane diisocyanate (t-CHDJ). The presence of the trio! leads to branching and/or crosslinking in the polyurethane/urea elastomer. Such cross linking has the disadvantage of increasing the viscosity of the prepolyrner and causing difficulties in processing. The addition of plasticizer to the prepolyrner reduces the viscosity thereof. The addition of plasticizer may be accomp\ ished during manufacture of the prepolyrner or thereafter. The resulting crosslinked, plasticized polyurethane/urea elastomers of this invention have improved thermal stability up to about 140-150°C and retain low temperature flexibility to about -35-( -40)°C. The thermally stable belts ofthe present invention, including timing or synchronous endless belts or belting, V-belts, multi-V-ribbed belts, micro-ribbed belts, flat belts or belting and the like, utilize the polyurethane/urea elastomers of this invention as their main body portion, and are manufactured using established belt fabrication methods. Unexpectedly, the belts of the present invention exhibit superior flex fatigue resistance and composite integrity compared to belts of non-plasticized and/or non-crosslinked, polyurethane/urea, even though plasticization and/or crosslinking result in certain reduced physical properties. [0027] Tlte present invention utilizes a two-step (prepolymer) approach via standard molding processes for preparing the polyurethane/urea elastomers. In the first step, a polyol mixture and the polyisocyanate react to yield the polyisocyanate prepolymer. In the second step, the polyisocyanate prepolymer and the chain extender react to produce the final polyurethane/urea elastomers. The plasticizer may be added to the prepolymer either before reacting the prepolymer or after the reaction. The amount of plasticizer in the prepolymer composition may be from about 5% to about30% by weight 8 based on the total prepolymer composition weight, preferably from 5% to about 15% by weight. [0028] The plasticizer for the present invention may be selected from the group consisting of dialkyl-ether di-alkylesters and polyalkylene-ether di-alkylesters, such as dior poly-ethylene glycol di-alkylesters. Dialkyl-ether diesters include C4 to C12-esters of C1- to C4-ether- or polyether-dicarboxylic acids. Examples of such plasticizers may include esters such as caprate, caprylate, hexanoate, heptanoate, pelargonate, 2- ethylhexoate,_ and the like. Examples of such plasticizers may include di-alkylesters of ethers suchras-ethylene glycol, propylene glycol, triethylene glycol, tetraethylene glycol, and polyethylene glycols having a molecular weight of up to about 800. A preferred plasticizer is polyethylene glycol di-2-ethylhexoate of molecular weight from about 300 to about 700. An exemplary plasticizer is PEG 400 di-2-ethylhexoate, having a molecular weight of 662 and sold under the trademark TegMeR® 809 by The Hai!Star Company. PEG 400 refers to a polyethylene glycol of molecular weight of about 400, or PEG with an average number of ethylene oxide units of about 8 or 9. [0029] The organic polyisocyanates suitable for the polyisocyanate prepolyrners used in the present invention are those possessing the following characteristics: compact and symmetric structure for aromatic compounds, or trans or trans, trans geometric structure for aliphatic compounds, for improved phase separation of the resulting elastomers, and high reactivity with amine groups to eliminate the need for catalysts in the formulations, which otherwise accelerate reversion of the resulting elastomers at high temperatures. The polyisocyanates useful as starting components for the preparation of the pol)