c*****e
发帖数: 238 | 1 Frustration effects in polymer systems
这个东西与我做的很有关系,所以听的比较认真.
所谓frustration,指的是体系有一个比较"喜欢"的低能量状态,但由于其他因素并
不能达到能量最低的状态.
一个简单的例子,如Ising magnet,或者不相容的polymer mixture,这种体系有相分离
(相变)的趋势,但在crosslink的A,B混合物中,相分离就不一定能顺利进行.比如
一般体系发生相分离的条件大概是
\chi * N～1,
在crosslink体系中,尽管总的趋势是满足这个条件的,但局部条件并不能得到满足
因为\chi *Ne(有效链长)<1.所以相分离不能顺利进行,体系就有可能被冻结在一个
metastable state.
具体的例子就是体系发生microphase separation的条件与具体的microphase structure
有关,并不能直接从mean field result来预言.
一般来说体系的相分离由concentration fluctuation诱发,但局部的fluctuation
有时并不一定 | w********h
发帖数: 12367 | 2
structure
这个concentration fluctuation有实验直接证明么？
的block的平均长度决定,在这种近似下每一种segment的block的等效长度大概是N^(1/2)
【在 c*****e 的大作中提到】
: Frustration effects in polymer systems
: 这个东西与我做的很有关系,所以听的比较认真.
: 所谓frustration,指的是体系有一个比较"喜欢"的低能量状态,但由于其他因素并
: 不能达到能量最低的状态.
: 一个简单的例子,如Ising magnet,或者不相容的polymer mixture,这种体系有相分离
: (相变)的趋势,但在crosslink的A,B混合物中,相分离就不一定能顺利进行.比如
: 一般体系发生相分离的条件大概是
: \chi * N～1,
: 在crosslink体系中,尽管总的趋势是满足这个条件的,但局部条件并不能得到满足
: 因为\chi *Ne(有效链长)<1.所以相分离不能顺利进行,体系就有可能被冻结在一个
| c*****e
发帖数: 238 | 3 这个只是一个一般的观点吧,实验可以测出concentration fluctuation,但不能确定
是否能引发phase separation/transition.
实际的情况是,在spinodal以后,任何微小的fluctuation都可以导致phase transition,
只要在q*有fluctuation.
但在亚稳区就不是一样了,通常需要比较大的fluctuation才能越过barrier.
所以要确定实际的phase transition point,就必须考虑fluctuation.
事实上,正是因为fluctuation任何体系才会回到equilibrium,或者到能量的最低态.
【在 w********h 的大作中提到】
:
: structure
: 这个concentration fluctuation有实验直接证明么？
: 的block的平均长度决定,在这种近似下每一种segment的block的等效长度大概是N^(1/2)
| w********h
发帖数: 12367 | 4
what I mean is:
can the idea (or theory) of "concentration fluctuation" be VERIFIED
experimentally? Though this idea is well accepted nowadays.
【在 c*****e 的大作中提到】
: 这个只是一个一般的观点吧,实验可以测出concentration fluctuation,但不能确定
: 是否能引发phase separation/transition.
: 实际的情况是,在spinodal以后,任何微小的fluctuation都可以导致phase transition,
: 只要在q*有fluctuation.
: 但在亚稳区就不是一样了,通常需要比较大的fluctuation才能越过barrier.
: 所以要确定实际的phase transition point,就必须考虑fluctuation.
: 事实上,正是因为fluctuation任何体系才会回到equilibrium,或者到能量的最低态.
| c*****e
发帖数: 238 | 5 好像是有通过light scattering来测量structure factor的,在spinodal附近确实有peak
在q*,大概算是测定fluctuation的大小吧.
【在 w********h 的大作中提到】
:
: what I mean is:
: can the idea (or theory) of "concentration fluctuation" be VERIFIED
: experimentally? Though this idea is well accepted nowadays.
| z**h
发帖数: 224 | 6 neutron scattering
factor的,在spinodal附近确实有peak
定
transition,
.
【在 c*****e 的大作中提到】
: 好像是有通过light scattering来测量structure factor的,在spinodal附近确实有peak
: 在q*,大概算是测定fluctuation的大小吧.
| z**h
发帖数: 224 | 7 diblock copolymer phase separation happens when chiN>10.5 from mean filed
theory.
structure
的block的平均长度决定,在这种近似下每一种segment的block的等效长度大概是N^(1/2)
【在 c*****e 的大作中提到】
: Frustration effects in polymer systems
: 这个东西与我做的很有关系,所以听的比较认真.
: 所谓frustration,指的是体系有一个比较"喜欢"的低能量状态,但由于其他因素并
: 不能达到能量最低的状态.
: 一个简单的例子,如Ising magnet,或者不相容的polymer mixture,这种体系有相分离
: (相变)的趋势,但在crosslink的A,B混合物中,相分离就不一定能顺利进行.比如
: 一般体系发生相分离的条件大概是
: \chi * N～1,
: 在crosslink体系中,尽管总的趋势是满足这个条件的,但局部条件并不能得到满足
: 因为\chi *Ne(有效链长)<1.所以相分离不能顺利进行,体系就有可能被冻结在一个
| c*****e
发帖数: 238 | 8 That is from accurate calculation.
Here we just estimate the limit.
【在 z**h 的大作中提到】
: diblock copolymer phase separation happens when chiN>10.5 from mean filed
: theory.
:
: structure
: 的block的平均长度决定,在这种近似下每一种segment的block的等效长度大概是N^(1/2)
| c*****e
发帖数: 238 | 9 Forgot to mention that this criterion is for symmetric diblock copolymers..
【在 c*****e 的大作中提到】
: That is from accurate calculation.
: Here we just estimate the limit.
| c*****e
发帖数: 238 | 10 I think dynamic/static light scattering is used for determination of the
structure factor for density fluctuation in phase transitions,
usually at the length scale of the wavelength of light.
X-ray scattering is used to determine the crystal lattice constants.
Neutron scattering could be used to determine the correlation functions of
deutrium, mostly used to characterize the conformation of a single chain,
what I know about SANS is it can be used to measure Rg.
【在 z**h 的大作中提到】
: diblock copolymer phase separation happens when chiN>10.5 from mean filed
: theory.
:
: structure
: 的block的平均长度决定,在这种近似下每一种segment的block的等效长度大概是N^(1/2)
| | | z**h
发帖数: 224 | 11 no, it is not only for symmetric. for symmetric diblock copolymer, lamellae
morphology will form after phase separation. with the change of volume
fraction of two blocks, the morphology will change also.
filed
的block的平均长度决定,在这种近似下每一种segment的block的等效长度大概是N^(1/2)
【在 c*****e 的大作中提到】
: Forgot to mention that this criterion is for symmetric diblock copolymers..
| z**h
发帖数: 224 | 12 the difference between the light anb x-ray or neutron is the length scale
they can measure. they all can measure the correaltion. just use small angle
scattering. usulay people use neutron because the contrast is bigger than
x-ray.
【在 c*****e 的大作中提到】
: I think dynamic/static light scattering is used for determination of the
: structure factor for density fluctuation in phase transitions,
: usually at the length scale of the wavelength of light.
: X-ray scattering is used to determine the crystal lattice constants.
: Neutron scattering could be used to determine the correlation functions of
: deutrium, mostly used to characterize the conformation of a single chain,
: what I know about SANS is it can be used to measure Rg.
| c*****e
发帖数: 238 | 13 For asymmetric diblock copolymer melts, the phase transition point from
mean field approximation is not \chi*N>10.495, but is higher.
The 10.495 limit is only at the critical point which is for symmetric diblock
copolymers.
【在 z**h 的大作中提到】
: the difference between the light anb x-ray or neutron is the length scale
: they can measure. they all can measure the correaltion. just use small angle
: scattering. usulay people use neutron because the contrast is bigger than
: x-ray.
| c*****e
发帖数: 238 | 14 It's correct that these methods are only different in the length scale.
But I never heard of using SANS to measure the parameters of microphases,
they are not of the same length scale.
【在 z**h 的大作中提到】
: the difference between the light anb x-ray or neutron is the length scale
: they can measure. they all can measure the correaltion. just use small angle
: scattering. usulay people use neutron because the contrast is bigger than
: x-ray.
| z**h
发帖数: 224 | 15 yes. for asymmetric it is bigger than 10.495
diblock
lamellae
copolymers..
的block的平均长度决定,在这种近似下每一种segment的block的等效长度大概是N^(1/2)
【在 c*****e 的大作中提到】
: For asymmetric diblock copolymer melts, the phase transition point from
: mean field approximation is not \chi*N>10.495, but is higher.
: The 10.495 limit is only at the critical point which is for symmetric diblock
: copolymers.
| z**h
发帖数: 224 | 16 nuetron reflecticity can measure the lamellae spacing.
the length between 10A and 1000A or bigger can be measured by neutron
angle
of
chain,
【在 c*****e 的大作中提到】
: It's correct that these methods are only different in the length scale.
: But I never heard of using SANS to measure the parameters of microphases,
: they are not of the same length scale.
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