HoaLyCacHopChat-c6

8/14/2019 HoaLyCacHopChat-c6

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Chng 6

c s ho l cc hp cht cao phn t

6.1. S m m d o c a m ch polyme Mt vt liu d rn nh th no, nhng khi t l gia chiu di v chiu ngang rt ln

th c kh nng un gp ln, th hin s uyn chuyn, ngha l th hin tnh mm do camnh. Polyme l vt liu m cu to phn t ca n gm hng nghn, hng vn nguyn t(nhm nguyn t) hp thnh, c kch thc chiu di ln hn rt nhiu kch thc chiungang, v vy th hin tnh mm do rt cao.

Khi nim v tnh mm do l khi nim u tin v c bn nht cho php gii thchcc tnh cht ho l khc thng ca polyme.

V d: Phn t polyetylen mch thng c cu to bi cc mt xch c s(-CH2- CH2-) c sp xp nh sau :

nCH2=CH2 (-CH2- CH2-)n

CH2 CH2 CH2 CH2CH2 CH2 CH2 CH2

Nu khi lng phn t trung bnh ca polyetylen t gi tr 104 - 105 th s mt xchc s (n) c gi tr trong khong 4.102 4.103. Mt phn t c kch thc chiu di ln hnrt nhiu so vi chiu ngang nh vy th do s chuyn ng nhit ni phn t m phn t s b un cong thnh nhiu hnh dng khc nhau, ngha l th hin tnh mm do.

Trong mch polyme, lin kt C - C khng cn tr phn ny ca phn t quay i viphn kia xung quanh phng ca lin kt l phng ca trc quay. Nh c s thay i hnhdng ca cc i phn t hydrocacbon m mt s polyme mch thng c th thc hin suyn chuyn ca mnh bng cch quay cc phn ring l ca cc nguyn t xung quanh

phng kt hp ca cc lin kt ho hc. S quay ca cc nguyn t theo phng lin ktc gi l s ni quay.

Nh vy tnh mm do ca mch polyme v bn cht chnh l s ni quay ca phnt. V vy trc khi xt cc tnh cht ca s mm do ca polyme chng ta hy tm hiu v sni quay phn t.

6.1.1. S ni quay phn t

Trong mi mt xch c s ca mch polyme, ngoi cc nguyn t to nn mch chnh(mch di) cn c cc nguyn t hay nhm nguyn t to nn mch nhnh (mch ngang)thng l nguyn t hydro hay cc nguyn t hoc nhm nguyn t thay th cho nguyn thydro. V th khi lin kt C - C ca mch chnh quay tng i th khong cch gia ccnguyn t mch nhnh b thay i. M nng lng tng tc tng h gia cc nguyn thay nhm nguyn t ph thuc vo khong cch ca chng, nn khi s ni quay xy ra thnng lng ca phn t b thay i. Ngha l mun s ni quay xy ra cn cung cp nnglng, nng lng cn cung cp c gi tr ph thuc vo v tr tng i ca cc nguyn ttrong phn t. Nu nng lng cn cung cp cng ln th s ni quay cng kh xy ra v phnt cng t th hin tnh mm do.

hiu r hn s ni quay ca phn t polyme, trc ht ta xt s ni quay ca phnt etan v cc dn xut ca n ri t suy ra s ni quay ca phn t polyme.

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1. S ni quay ca phn t etan

Trong phn t etan (C2H6), hai nguyn t cacbon lin kt vi nhau v vi cc nguynt hydro bng lin kt cng ho tr, to thnh 2 hnh t din, gc ho tr to nn bi cc linkt bng 109028' v c m hnh cu to nh sau :

Nguyn t H

Nguyn t C

Hnh 6.1. M hnh cu to ca phn t etanTrc y Van't Hoff gi thit rng gia cc nguyn t khng trc tip lin kt ho

hc vi nhau th khng c s tng tc, ngha l v tr khng gian ca chng (c c trngbi gc quay ) hon ton khng nh hng n th nng tng tc (U) ca phn t, U =const (hnh 6.2). Khi hai hnh t din ca phn t etan c th quay t do xung quanh linkt C - C v s quay nh vy c gi l s ni quay t do. y l trng hp l tng.

U

Hnh 6.2. S ph thuc ca th nng tng tc U vo gc quay trong trng hp l tng

Nhng thc t th gia cc nguyn t khng lin kt ho hc vi nhau vn c stng tc tng h. Bn thn cc lc tng tc gia cc nguyn t c th khc nhau, tuthuc vo cu to nguyn t trong c cc loi tng tc nh: tng tc ion, tng tc nh

hng, tng tc bin dng, tng tc hydro...Phn t trng thi chuyn ng, hnh thc chuyn ng c th l s dao ng ca

cc nguyn t xung quanh trung tm cn bng hay s quay ca cc nhm nguyn t xungquanh lin kt ho tr... Do s chuyn ng nhit m v tr khng gian ca cc nguyn t lunlun thay i, th nng mi v tr khng gian ca nguyn t khng bng nhau do s tngtc tng h ca nguyn t khng lin kt ho hc vi nhau.

Xt phn t etan ta thy: do s quay ca hai nhm -CH 3 xung quanh lin kt ho tr C- C nn th nng ca phn t lun lun thay i. Nu m t s ph thuc ca th nng vogc quay bng hm U = f() (: gc quay) ta c kt qu sau

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B ng 6.1. S t ng quan gi a gc quay, c u hnh v th n ng c a phn t etan.

Gc quay Cu hnh Th nng U

0U1 ln nht, v cc nguyn t -H gn nhau nht nnlc tng tc tng h ln.

60U2 < U1, v cc nguyn t -H xa nhau hn nn lctng tc tng h yu hn.

120 U3 = U1

180

U4 = U2

240 U5 = U1

300 U6 = U2

360 U7 = U1

U

U1

U2

0 60 120 180 240 300 360 Gc quay ()Hnh 6.3. S bin thin th nng ca phn t etan vo gc quay

Gi tr Uo = Umax - Umin (i vi trng hp ca etan th U o = U1 - U2) c gi lnng lng hot ho ca s quay hay hng ro th nng ca s quay. Nu gi tr U o cng bth s ni quay cng d xy ra v ngc li.

2. S ni quay ca phn t dicloetan Cl-CH2-CH2-Cl

S ni quay ca dicloetan tng t nh trng hp ca phn t etan, nhng ch ivi phn t dicloetan th tng tc tng h gia 2 nguyn t clo l ln hn. Ta c cc kt

qu nh sau:

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B ng 6.2. S t ng quan gi a gc quay, c u hnh v th n ng c a phn t dicloetan.

Gc quay Cu hnh Th nng U

0

U1 b nht, v cc nguyn t khng lin kt xanhau nht nn lc tng tc tng h km nht.

60

U2 > U1, v cc nguyn t gn nhau hntrng hp 1.

120

U1 < U3 < U2, v khong cch gia cc nguynt nh trng hp 1, nhng 2 nguyn t clo

gn nhau hn

180U4 ln nht, v cc nguyn t cng loi gnnhau nht

240 U5 = U3

300 U6 = U2

360 U7 = U1

U

U4

U2

U3

U1

0 60 120 180 240 300 360 Gc quay ()

Hnh 6.3. S bin thin th nng ca phn t dicloetan vo gc quay

Thc nghim chng minh rng: ng phn ca phn t dicloetan nhit xcnh bn hn ng phn ca phn t etan. S chuyn t ng phn quay ny n ng phn

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quay khc i hi tiu tn mt nng lng nht nh gi l nng lng hot hot ho hayhng ro th nng ca s quay. Ngun nng lng ny c th l nhit nng hay lc tc ngc hc no ln h. Kt qu tnh ton cho thy nng lng hot ho ca phn t etan bng2,75 kcal/mol nhit phng (20oC), phn t etan d t ti mc nng lng ny, do sni quay phn t etan tng i t do.

C th xt s ni quay phn t ca polyme tng t s ni quay ca phn t etan. Vd i vi polyetylen. Ta c th xem i phn t polyetylen nh l mt dn xut ca etan :

...-CH2- CH2- CH2- CH2- CH2- CH2- CH2- CH2-...

R1 R2

Tuy nhin trong trng hp ny s ni quay ca i phn t polyme phc tp hn, v2 nhm R1, R2 l nhng mch di v phc tp, nn tng tc tng h gia chng r rng la dng hn.

6.1.2. S mm do ca mch polyme

1. S m m d o nhi t ng h c Gi s v tr 1, th n ng c a phn t l U 1, khi chuy n

sang v tr 2, th n ng c a phn t l U 2. Bi n thin n ng l ng khi h chuy n t v tr 1 sang v tr 2 l :

U = U2 - U1

i l ng U c tr ng cho s m m d o nhi t ng h c. N cho bi t kh n ng c a phn t c th th c hi n c s chuy n ho hnh d ng hay khng ( ngh a c a U y gi ng ngh a c a i l ng bi n thin th ng p ng nhi t G khi xem xtkh n ng c a m t ph n ng ho h c). N u U < 0 th s chuy n ho hnh d ng c a phn t c th x y ra, ngh a l polyme c kh n ng th hi n tnh m m d o.

2. S mm do ng hc

Khi tho mn iu kin mm do nhit ng hc U < 0, ngha l polyme c khnng th hin s mm do, nhng tc ca s chuyn ho hnh dng li ph thuc vo ilng Uo = Umax - Umin..Uo c trng cho s mm do ng hc. N cho bit tc ca qutrnh chuyn ho hnh dng (tng t gi tr nng lng hot ho Ea trong phn ng ho hc).

Nu gi tr Uo cng ln th tc chuyn ho hnh dng cng chm, polyme cng t th hintnh mm do.

U

Uo

U1

U2

Hnh 6.4. Nng lng hot ho ca s ni quay phn t polyme

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lm r kh n ng chuy n ho hnh d ng (hay kh n ng th hi n tnh m m d o) c a polyme, Gut v Mak a ra khi ni m "C u d ng" m t v tr khng gian c a cc nguyn t (hay nhm nguyn t ) trong phn t .

C u d ng l v tr khng gian c a nguyn t hay nhm nguyn t c m c n ng l ng kh cao, do s n i quay gy nn d i tc d ng c a ngo i l c nh nhi t n ng, c n ng... C u d ng th c ra l xc su t phn b c a cc nguyn t (nhm nguyn t ) trong phn t do s chuy n ng nhi t c a phn t gy nn. Khc v i c u hnh l v tr khng gian c nh c a cc nguyn t (nhm nguyn t ) trong phn t , v d c u hnh cis , trans, c u hnh izotactic, syndiotactic. C u hnh phn t ch thay i khi tc ng ln h m t n ng l ng b gy lin k t ho h c c a phn t .

i vi i phn t ca polyetylen, ta thy rng cc nhm -CH2- ca phn t lun lun

quay xung quanh trc lin kt CC. Gi thit rng s ni quay xy ra hon ton t do cngha l cc gc ho tr khng c nh. V cc gc ho tr khng c nh nn cc mt xch cth bt k v tr khng gian no, khng ph thuc vo cc mt xch bn cnh. Ni cch khcs cu dng ca phn t t ti cc i. Mch phn t nh vy th hin mm do ti a.

Trong thc t do c s tng tc tng h m gc ho tr b bin dng trong qu trnhquay. Xt mch polyetylen. Nu qui c mt xch u tin nh hng theo trc tung, th mtxch th hai s nh hng xung quanh mt hnh nn lm vi trc ca mt xch th nht mtgc 109o28', v mt xch th ba tng t s nh hng xung quanh mt hnh nn lm vitrc ca mt xch th hai mt gc 109o28', s nh hng ca cc mt xch tip theo l tngt.

Hnh 6.5. S nh hng ca cc mt xch trong polyme

Nh vy hng trc quay ca mc xch th hai cn ph thuc nhiu vo hng ca

mt xch th nht, hng ca mt xch th ba li ph thuc vo hng ca mt xch th hainhng ph thuc t hn vo hng mt xch th nht... C suy lun nh vy, th s n mtmt xch no m hng trc quay ca n khng cn ph thuc vo hng ca mt xchu tin na. Mt chui mt xch c tnh cht nh vy c gi l on mch.

Nh vy on mch l i lng bao gm mt s mt xch, m nh hng s niquay ca mt xch cui cng khng ph thuc vo hng quay ca mt xch u tin. S mtxch trong mi on mch cng t th mch biu hin tnh mm do cng cao. Trong cctrng hp gii hn, khi kch thc on mch bng kch thc mt mt xch th mch

polyme c mm do l tng, ngc li nu kch thc on mch bng kch thc camch polyme th polyme khng th hin tnh mm do hay c cng ln nht. Trong trng

hp ny, mch dui thng, s cu dng bng 1. V vy c th ni : "on mch l thc o smm do ca mch polyme".

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Gi tr on mch ch l gi tr tng tng, c hnh dung ra gii thch bn chtmm do ca mch polyme. Gi tr on mch ph thuc vo bn cht ca polyme, vo mitrng, ngun nhit nng cung cp cho h, trng lc tc dng ln h.

6.2. cc yu t nh hng n s mm do ca mch polyme

6.2.1. Kch thc v phn cc ca nhm thXt cc polyme c tng hp t monome dng vinyl CH2=CHX. Nu nhm th -X

c kch thc cng ln v cng phn cc th s tng tc tng h gia chng cng ln, do s lm gim tnh mm do ca mch polyme.

Nu snh tnh mm do ca polyetylen (PE), polystyren (PS) v polyvinylclorua(PVC), th tnh mm do c sp xp theo th t gim dn nh sau:

Tnh mm do ca PE > PVC > PS

(-CH2-CH)n (-CH2-CH-)n (-CH2-CH-)n| | |

H Cl C6H56.2.2. Nhit

Nhit cng cao th polyme cng th hin tnh mm do r hn, bi v khi s niquay c iu kin xy ra d dng hn. Cc nguyn t, nhm nguyn t c cung cp nnglng thng lc tng tc tng h, vt qua hng ro th nng thc hin s ni quayv do th hin tnh mm do ca mch polyme.

6.2.3. Mt mng li khng gian

i vi polyme c cu trc mng li nu mt mng li khng gian (s cc linkt cu ni gia cc mch) cng ln th polyme cng km mm do. S d nh vy l v gia

cc nguyn t, nhm nguyn t trong mch by gi c tng cng cc lin kt vi nhau viu ny hn ch s ni quay ca phn t ngha l lm gim tnh mm do ca mch polyme.

Khi lu ho cao su thin nhin, nu hm lng lu hunh (S) t 1 - 3% th cao su luho c tnh n hi tt, nhng nu cng tng hm lng S th s mm do s gim. Khi hmlng S > 30% th cao su lu ho khng th hin tnh mm do m tr nn cng, gin.

Nh vy on mch l i lng bao gm mt s mt xch, m nh hng s niquay ca mt xch cui cng khng ph thuc vo hng quay ca mt xch u tin. S mtxch trong mi on mch cng t th mch biu hin tnh mm do cng cao. Trong cctrng hp gii hn, khi kch thc on mch bng kch thc mt mt xch th mch

polyme c mm do l tng, ngc li nu kch thc on mch bng kch thc ca

mch polyme th polyme khng th hin tnh mm do hay c cng ln nht. Trong trnghp ny, mch dui thng, s cu dng bng 1. V vy c th ni : "on mch l thc o smm do ca mch polyme".

6.3. ng cong c nhit ca polyme v nh hnh

6.3.1. ng cong c nhit

Nu tc dng ln vt th mt lc ko f khng i (f = const) vi tc lc tc dngkhng i trong khong nhit nht nh th vt th s b thay i hnh dng, ngi ta givt b bin dng.

bin dng () c xc nh theo biu thc: oo

l

ll =

.100%

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vi lo, l l chiu di ca vt th trc v sau khi tc dng lc

th biu din s ph thuc ca bin dng vo nhit c gi l ng congc nhit.

Tu thuc bn cht ca vt th chu tc dng lc ta c cc trng hp sau:

1. Nu vt th l hp cht thp phn t tinh thy chnh l qu trnh chuy n pha, v t th chuy n t

tr ng thi r n sang tr ng thi l ng. ng cong c nhi t c d ng hnh 6.6a. Qu trnh chuy n pha x y ra t i m t nhi t nh t nh g i l nhi t nng ch y. Qu trnh chuy n pha lm cho c u trc c a v t th thay i, cc thng s nhi t ng c a h nh th tch ring, nhi t dung, entropi, n i n ng ... thay i m t cch t ng t.

(%) (%)

Tnc T (0C) Tg T (0C)

(a) (b)

Hnh 6.6. ng cong c nhit ca vt th thp phn t

vng tinh th s bin dng ca vt th l thun nghch v tun theo nh lut Hooke :

EE

1.

S

f ==

vi : ng sut

E : mun n hi

S : tit din chu tc dng ca lc f

Sau im nng chy, s bin dng tng nhanh theo nhit v tr thnh bt thunnghch. S bin dng trong vng ny tun theo nh lut Newton :

t.1

=

vi : nht ca h

t : thi gian tc dng lc

2. Nu vt th l hp cht thp phn t v nh hnh

y l qu trnh vt th chuyn t trng thi thu tinh sang trng thi chy (khngphi l qu trnh chuyn tng). Trng thi thu tinh v trng thi chy gii hn vi nhau binhit thu tinh Tg. Nhit thu tinh Tg thng dao ng trong gii hn 10 - 20 . ilng bin dng thay i u v lin tc tng ng vi s thay i nht ca h. Nhvy, qu trnh thu tinh ho l qu trnh vt th chuyn t trng thi vt l ny sang trng thivt l khc nhng h nm trong tng lng. Mt s hp cht thp phn t ch tn ti trngthi thu tinh nh nha thng, thu tinh silicat... (hnh 6.6b)

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3. Nu vt th l polyme v nh hnh

(%)

vng I vng II vng III

Tg Tc T (0C)

Hnh 6.7. ng cong c nhit ca polyme v nh hnh

ng cong c nhit ca polyme v nh hnh c m t hnh 6.7. Ngoi trng thithu tinh v trng thi chy xt trn cn xut hin mt trng thi mi l trng thi co dn

(n hi), trng thi n hi nm gia trng thi thu tinh v trng thi chy. Trng thi nych biu hin cc vt th polyme c khi lng phn t ln nh cao su thin nhin, cao sutng hp. c tnh ca trng thi ny l c kh nng bin dng thun nghch ti 10 - 15 lntrong khong nhit tng i rng vi gi tr mun n hi tng t nh mun n hica cht kh. Cn lu rng cc trng thi vt l c bn ca polyme v nh hnh ch tn titrong tng lng, ph thuc vo nhit . Polyme v nh hnh d dng chuyn t trng thithu tinh sang trng thi n hi v trng thi chy hoc ngc li. Do khi lng phn t lnv do tnh mm do ca mch polyme, hin tng hi phc biu hin rt r trong qu trnh

bin dng tt c ba trng thi vt l ca h. Tht vy, nhit thu tinh v nhit chykhng phi l nhit xc nh, ph thuc vo tc bin dng v tc nung nng hay lmlnh m Tg v Tc c khi dao ng trong khong 20 - 30 .

6.3.2. nh hng ca khi lng phn t (KLPT) n dng ng cong c nhit capolyme v nh hnh

Xy dng ng cong c nhit ca nhiu mu polyme c cng bn cht nhng KLPTkhc nhau : M1 < M2 < M3 < M4 < M5 < M6.

Nu biu din trn cng mt th th ta c :

(%)

M1 M2 M3 M4 M5 M6

Tg Tc T (0C)

Hnh 6.8. ng cong c nhi t c a polyme v nh hnh c KLPT khc nhau.

Phn tch cc ng cong c nhi t trn hnh 6.8 ta th y:

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- Khi khi lng phn t cn thp (ng vi cc ng M 1, M2, M3) th vt th ch tnti hai trng thi l trng thi thu tinh v trng thi chy tng t nh dng ng congc nhit ca vt th thp phn t, ngha l v tnh cht c hc chng khng khc g cc sn

phm thp phn t. Trong trng hp ny khi lng phn t tng ch lm tng nhit thutinh Tg (Tg[M3] > Tg[M2] > Tg[M1]) .

- Khi gi tr khi lng phn t M4 th ng cong c nhit mi bt u xut hintrng thi co dn, polyme mi th hin tnh n hi. Bt u t gi tr khi lng phn t ny(M4) th s tng khi lng phn t khng nh hng n nhit thu tinh Tg m ch nhhng n nhit chy Tc. Khi lng phn t cng ln th nhit chy T c cng tng,ngha l vng n hi cng rng.

S tng nhit chy khi khi lng phn t tng c th gii thch nh sau: s chych c th xy ra khi c s dch chuyn bt thun nghch ca phn t theo chiu lc tc dng.Khi khi lng phn t tng th nng lng tng tc tng h gia cc phn t cng tng do phi tng nhit khc phc nng lng tng tc tng h ca phn t cn trngthi co dn. n v chuyn ng nhit l on mch, linh chuyn ng nhit ca on

mch ph thuc vo kch thc ca phn t, do khi khi lng phn t bng hoc ln hnon mch th mch s biu hin tnh mm do ca n v trn ng cong c nhit xut hintrng thi co dn. T ta c th nh ngha: on mch l gi tr khi lng phn t ti thiu mch polyme biu hin tnh mm do ca mnh.

Mt s polyme c cu trc c bit, lin kt gia cc phn t ca mch polyme rtbn, nng lng tng tc tng h gia cc phn t rt ln, vt th khng biu hin tnh codn, trong trng hp ny nhit thu tinh ln hn nhit phn hu, polyme s b phnhu trc khi un nng n n nhit ny. Xenluloz l mt v d cho loi polyme ny.

Cng c trng hp, nhit phn hu ca polyme ln hn nhit thu tinh nhngli nh hn nhit chy. Polyme c th chuyn sang trng thi n hi cao, nhng khi

un nng tip tc n s b phn hu m khng chuyn sang trng thi chy c. ltrng hp i vi cc polyme mng li.

Nh vy, khng phi tt c cc polyme v nh hnh u c th tn ti c 3 trngthi v ch khi kch thc ca mch polyme kch thc on mch th polyme mi th hintnh mm do, khi ng cong c nhit mi xut hin vng n hi.

ngha ca vic kho st ng cong c nhit:

Kh o st s ph thu c d ng ng cong c nhi t vo kh i l ng phn t c a polyme cho php chng ta :

1. Xc nh khi lng phn t ca polyme bng phng php c nhit thun tu

m khng cn phi ho tan polyme.2. Cho bit polyme c th tn ti nhng trng thi no v nhit no th cc

trng thi xut hin. Da trn nhng kt qu cho php chng ta kt lun khnng s dng polyme trong iu kin nhit v ng hc no, cng nh cc iukin khi gia cng, bin tnh polyme.

6.4. S bin dng v hi phc

6.4.1. n hi sau lc tc dng

Khi tc dng ln mu polyme trng thi cn bng mt lc f th cc qu trnh t dinbin xy ra trong h s a h ra khi trng thi cn bng c v chuyn h sang trng thi cn

bng mi. Tng ng vi lc tc dng, phi mt mt thi gian nht nh (trong nhiu trnghp qu trnh ny c th ko di hng gi, hng ngy hoc thm ch hng nm) qu trnh t

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din bin trn xy ra. V vy, ta khng nhng theo di c trng thi ban u v trng thicui ca h m c th theo di c c qu trnh trung gian.

Tin hnh th nghim nh sau: c nh mt u ca mu polyme, u cn li c treomt qu cn, khi chiu di ca mu polyme s tng dn, ta ni rng mu polyme b bindng. Qu trnh bin dng xy ra chm v cui cng t ti mt tc xc nh. Khi b lctc dng, th qu trnh xy ra ngc li vi s bin dng, mu dn dn quay li trng thi banu vi tc xc nh, ngha l c s hi phc ca polyme sau khi thi lc tc dng cangoi lc.

Nu biu din s ph thuc ca bin dng tng i ca polyme v nh hnh vothi gian khi tc dng mt lc ko cng c hc khng i bng ng lin nt v s hi phckhi thi lc tc dng bng ng t nt ca 2 mu polyme: mch thng v mch khu (cutrc mng li) trn cng mt th, ta c :

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bin dng ()

1max

(1)

2

max (2) 1

t1 t2 Thi gian (t)

Hnh 6.9. S bin dng v hi phc ca polyme theo thi gian

ng (1) : i vi polyme mch thng

ng (2) : i vi polyme mng liNhn xt:

i vi polyme mch thng, s bin dng bao gm hai giai on:

- Giai on th nht do qu trnh dui thng ca cc phn t di tc dng ca ngoilc. Do phng lin kt gia cc nguyn t trong mch chnh khng trng nhau, cu trc camch polyme c dng gp khc hoc cun trn. Di tc dng ca ngoi lc s dui thng

phng lin kt ca cc nguyn t thnh mt ng thng v kt qu l mch polyme b dndi. Hp phn ny sau khi t gi tr xc nh 1 (ng vi thi gian t1) s khng i nu vntip tc chu tc ng ca ngoi lc. S bin dng nh vy gi l bin dng n hi. Nu

thi gian t1 ta thi tc dng lc th mu polyme s hi phc v hnh dng v kch thc banu.

- Giai on th hai do qu trnh chy, ti t1 nu vn tip tc tc dng lc ln mupolyme th cc phn t s b trt ln nhau. Lc ko cng tc dng cng lu th hp phn thhai cng ln, v t gi tr cc i max thi gian t2. S bin dng ny c gi l bin dngchy (bt thun nghch). Ti thi im t2 nu b lc tc dng th mu polyme ch hi phcc mt phn. Phn hi phc chnh l hp phn bin dng n hi. Do bin dngkhng gim v gi tr 0, m t gi tr nht nh.

i vi polyme c cu trc mng li (mch khu) khi chu tc dng ca ngoi lc thch c bin dng n hi xy ra, do bin dng cc i ca polyme mch khu nh hn

so vi polyme mch thng (2max


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ng sut =S

f

(1)

cb

(2)

Thi gian (t)

Hnh 6.10. ng cong hi phc ng sut theo thi gianng (1) : i vi polyme mch khung (2) : i vi polyme mch thng

Nu ko cng nhanh mt mu polyme v nh hnh n mt chiu di xc nh, th lc

ko c hc (c c trng bi ng sut ) cn thit gi c chiu di mu khng i sgim dn theo thi gian. i vi polyme mch thng th ng sut gim dn ti gi tr 0, cnnu polyme c cu trc mng li th ng sut gim dn v t ti gi tr ng sut cn bngno .

Phn tch qu trnh, chng ta thy rng :

- i vi polyme mch thng, thi im u, khi tc dng nhanh mt lc ln mu,th phn t khng kp dui ra theo chiu lc tc dng, trong khi lc tc dng cn lm thayi khong cch gia cc phn t v lm bin dng gc ho tr cho nn h n hi nh vt thrn i hi ng sut ln hn so vi trng hp bin dng co dn. V vy tc lc tc dngcng ln th phn t cng khng kp nh hng theo chiu lc tc dng. Cng v l do m

tc lc tc dng c th lm thay i bn cht n hi ca vt th. Gi tr mun n hiph thuc trc tip vo tc lc tc dng v tng theo s tng tc ca lc.

Sau khi ko cng nu vn gi mu trng thi cng th chuyn ng nhit lm cho cutrc ca mu b thay i dn dn, cc phn t kp nh hng v di chuyn theo hng calc tc dng, cho nn mu dn dn c dn di ra. Mun cho dn di khng thay i th

phi gim lc ko mt cch tng ng, hay ni cch khc ng sut lc ny gim dn. Khi lcko c duy tr trong mt khong thi gian ln th xy ra s dch chuyn khng thunnghch ca cc n v cu trc, ngha l xy ra s chy. Kt qu l lc ko cng cn thit duy tr dn di khng i ngy cng gim v i vi polyme mch thng va c bin dngn hi, va c bin dng chy nn cui cng ng sut gim n gi tr 0.

- i v i tr ng h p polyme m ch khu th khng th c qu trnh ch y, v gi a cc phn t lun lun c cc c u n i gi l i, do mu n duy tr tr ng thi dn di c a n ph i duy tr m t l c ko c h c khng i no tc d ng ln m u. V v y ng su t tuy c gi m nh ng ch t n m t gi tr ng su t cn b ng no ch khng th gi m n 0.

N u m t m ng l i cng dy c th gi tr t i h n c a ng su t cn b ng cng l n, v gi tr ny c ng l th c o m t m ng l i khng gian.

6.4.3. Hin tng n hi tr

Nu tc dng ln mu polyme mt lc ko cng c hc tng dn v sau gim dnlc ny vi tc nh th, th u tin mu s b bin dng v sau hi phc li hnh dng

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v kch thc ban u. Nu s bin dng lun lun t gi tr cn bng th ng biu din sbin dng v s hi phc phi trng nhau, ngha l c s ph thuc tuyn tnh gia ng sut

v bin dng theo nh lut Hooke:E

=

Tuy nhin trong thc t hai ng bin dng (1) v hi phc (2) khng trng nhau mto thnh vng, c gi l "vng tr".

bin dng ()

2

(2)

1 (1)

ng sut ()Hnh 6.11. Hin tng n hi tr

S khc nhau gia hai qu trnh bin dng v hi phc l do s bin dng din rachm hn so vi s thay i ca ng sut. Khi tng ng sut thc t s bin dng m ta theodi, quan st c lun lun nh hn s bin dng cn bng (c bit khi ng sut tng nhanhth s sai khc ny cng ln) v s bin dng i hi thi gian cn thit t gi tr cn bng.Cng v l do khi gim ng sut th gi tr bin dng xc nh c lun lun ln hngi tr bin dng cn bng. Thm ch khi ng sut gim n gi tr 0 m mu polyme vn cnmt gi tr bin dng 1.

Nh vy s thay i bin dng lun b chm hn s thay i lc ko cng. Dnhin mun tp hp cc phn t chui uyn chuyn cn phi c mt thi gian xc nh, chonn s ph thuc ca bin dng vo lc ko cng lc tng hay gim khng trng nhau. Vicng mt lc ko, bin dng khi tng lc lun lun nh hn bin dng khi gim lcko.

Din tch vng gii gia ng bin dng v ng hi phc c gi l "Din tchvng tr" (S). Din tch vng tr ph thuc vo cc yu t sau :

1. Tc lc ko

- Nu vn tc lc ko nh th s chnh lch gia bin dng thc t v bin dngcn bng b (v cc phn t c thi gian dui ra theo chiu lc tc dng) nn hai nhnh (1)

v (2) gn nhau hn, khi din tch vng tr nh.- Nu vn tc lc ko ln th s chnh lch gia bin dng thc t v bin dng

cn bng ln, hai nhnh (1) v (2) xa nhau hn, do din tch vng tr ln.

2. Nhit

- Nu nng cao nhit s lm cho s bin dng thay i nhanh hn v khi nhit cao th s bin dng theo kp s thay i lc ko nn c th xem s bin dng t ctrng thi cn bng, hai nhnh (1) v (2) rt gn nhau v din tch vng tr s rt nh.

- Khi gim nhit th vn tc sp xp li cc n v cu trc s gim, khi nhit qu b s bin dng hu nh khng xy ra theo thi gian tc dng ca lc ko cng tng dn,

lc ny din tch vng tr cng nh.

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Din tch vng tr s ln nht nhit trung gian, khi bin dng ca mu xy ra vimt vn tc r rt nhng chm hn so vi s thay i ca lc ko cng.

Din tch vng tr c mt ngha c bit, v n bng mt nng lng b khuchtn sau mt chu trnh bin dng. Din tch vng tr c biu din di dng tng ca 2 tch

phn :

S =

+1

2

2

dd2

o

1(6.1)

trong : 1 tng ng vi gi tr lc ko cng khi tng lc ko

2 tng ng vi gi tr lc ko cng khi gim lc ko

bin dng tng ioo

o

l

l

l

ll =

= hay d =

ol

dl

Mt cng khi bin dng (tiu tn hoc c b li khi mu dn ra hay hi phc li

mt on dl di tc dng ca lc F) c biu din bng phng trnh :

V

dl.F

l

dl.

S

Fd

o

== (6.2)

vi S l tit din ca mu chu tc dng ca ngoi lc

V l th tch mu

V vy tch phn th nht ca phng trnh (1) l cng ca qu trnh ko cng mubng ngoi lc tnh cho mt n v th tch ca mu (c gi tr dng); cn tch phn th hail cng hon li khi mu hi phc (c gi tr m).

Tng ca hai tch phn trong phng trnh (1) cho ta bit hiu s mt nng lngtiu tn v nng lng c b li. Din tch vng tr S cng ln th nng lng cn li trongmu polyme bin dng cng ln (sau khi kt thc mt chu trnh). Phn nng lng ny schuyn thnh nhit nng lm nng polyme. V vy s ph thuc ca din tch vng tr vovn tc lc ko cng v nhit cho php chng ta phn on v qu trnh to thnh nhit vv tn tht nng lng c hc khi bin dng polyme cc iu kin nhit v ng hckhc nhau.

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