The Gordon‐Taylor equation relating the glass transition temperature θ of a copolymer to the glass transition temperatures θ1 and θ2 of the homopolymers is equivalent to where c1 and c2 are the weight fractions of the constituents and A1 and A2 are constants. It can be recast into the following forms suitable for linear plots and where k = A2/A1. Data from the literature on 10 copolymer systems, including butaciene‐styrene copolymers, give linear plots, verifying the equation within experimental error. However, the observed value of k is in most cases significantly smaller than the ratio of the differences of the volume‐temperature coefficients for each homopolymer in the rubbery and glassy states, as required by the derivation of Gordon and Taylor. The glass transition temperature (in °C.) for a butadiene‐styrene copolymer prepared by emulsion polymerization at 50°C. may be calculated from the weight fraction c2 of bound styrene as and for a similar 5° copolymer as

The crystallization and melting of unvulcanized natural rubber in the unstretched state have been investigated at different temperatures. Change of volume has been used as a quantitative measure of the extent of crystallization, and mercury-filled dilatometers containing the rubber have been used for the volume measurements. Crystallization was observed to occur at temperatures between −50° and +15°C and to be most rapid at about −25°C. The final decrease of volume on crystallization was usually found to lie between 2.0 and 2.7 percent. The melting of the crystalline rubber was found to occur over a range of temperature and to be strongly dependent on the temperature at which the crystals were formed. The temperature at which the beginning of melting occurs is from 4° to 7° above the temperature of crystallization. The range of melting is about 35° at the lowest temperatures and decreases to about 10° at the highest. The same range of temperature of melting is obtained regardless of the extent of the crystallization.

In cold climates some rubber products have been observed to undergo slowly a great increase in rigidity and permanent set.

The specifi c volumes of unvulca nizcd natural rubbcr and of a peroxide-cured vulcan izate of natural rubber were meas ured at press ures of 1-500 kg/cm2 at temperatur cs from 0 to 25°C. Observations on mercury-filled d ilatometers were made through a window in t he press urc system. No t ime effects or hysteres is phenomcna were observed. The spec ific volumc 17 in cm 3 /g over t he range studi ed can be r epresen ted by 17= V o,2511 + A(t -25 ) 111 + [a 25 + k,(t -25 )whcre P is t he press ure in k g/cm2, and t tIl e tcmperatu re in °C. The constants for thc un v ulcanized and for t he peroxide-cured samples are: 170,25 = 1.0951 and 1.1032 cm 3 /g; 10'A = 6.54 and 6.36 pCI' degree ; 106a25= -50.5 and -50.4 (kg/ cm2)-'; 106k ,= -0. 227 a nd -0.203 per dcgrec ; 1091325 = 10 and 11.5 (kg/cm2)-2; and 109k 2= 0.O L 18 and 0.073 p CI' degree, r es pcctively. The oompr essibili ty of unvulcanizcd natural rubbcr at 25° a nd 1 k g/cm2 is t hus 50.5 X 10-6 (kg/Clu2) -1 fa llin g to 40.6 X 10-6 (kg/ cm2) -1 at a press lll'e of 500 kg/cm 2. It I S concluded that a low deg rce of vulcanization produces no ignifi ca n t chan ges in the constants lis ted. The valu cs a re no t far d iffcren t from those obtained by extrapolating to zero s ulfur co ntcnt the observa tio ns of Scott on t ile rubbersulfur system. Calcula tions of valu es of compressibility (a nd i ts r eciprocal thc bulk modulus), " in te rn al press ure", bulk wave vclocity, difference be twee n sp ecifi c heats and severa l other physical propert ies are in r easonable agree mcnt with those obtained by d i;'ect obser vat ion by oth er worke rs. For t he prcd ietion of valu es at press ures above 50'0 ko-/eln2 the use of the Tait eq uation is recomme nd ed.

Natural rubber crosslinked by dicumyl peroxide in amounts up to 25 parts per hundred of rubber (phr) showed a maximum in tensile strength near 1 phr, followed by a steep decrease to a minimum near 5 phr. The ultimate elongation decreased from 870% at 0.5 phr. to about 10% above 10 phr. The modulus increased linearly with increase of crosslinking. The creep rate decreased from 5.6% per decade at 0.5 phr to zero at 5 phr and higher values. Crystallization, with a resultant abrupt increase in creep, was noted in specimens held in the stretched condition for more than one day. Between 5 and 25 phr this system (when crystallization is avoided) appears to function as an ideal elastic network and can be recommended for studies of rubber elasticity since no variation of modulus with time is observed.